MySQL Contributions status

This post is an update to the status of various MySQL bugs (some with patches) that I’ve filed over the past couple of years (or that people around me have). I’m not looking at POWER specific ones, as there are these too, but each of these bugs here deal with general correctness of the code base.

Firstly, let’s look at some points I’ve raised:

  • Incorrect locking for global_query_id (bug #72544)
    Raised on May 5th, 2014 on the internals list. As of today, no action (apart from Dimitri verifying the bug back in May 2014). There continues to be locking that perhaps only works by accident around query IDs.Soon, this bug will be two years old.
  • Endian code based on CPU type rather than endian define (bug #72715)
    About six-hundred and fifty days ago I filed this bug – back in May 2014, which probably has a relatively trivial fix of using the correct #ifdef of BIG_ENDIAN/LITTLE_ENDIAN rather than doing specific behavior based on #ifdef __i386__
    What’s worse is that this looks like somebody being clever for a compiler in the 1990s, which unlikely ends up with the most optimal code today.
  • mysql-test-run.pl –valgrind-all does not run all binaries under valgrind (bug #74830)
    Yeah, this should be a trivial fix, but nothing has happened since November 2014.
    I’m unlikely to go provide a patch simply because it seems to take sooooo damn long to get anything merged.
  • MySQL 5.1 doesn’t build with Bison 3.0 (bug #77529)
    Probably of little consequence, unless you’re trying to build MySQL 5.1 on a linux distro released in the last couple of years. Fixed in Maria for a long time now.

Trivial patches:

  • Incorrect function name in DBUG_ENTER (bug #78133)
    Pull request number 25 on github – a trivial patch that is obviously correct, simply correcting some debug only string.
    So far, over 191 days with no action. If you can’t get trivial and obvious patches merged in about 2/3rds of a year, you’re not going to grow contributions. Nearly everybody coming to a project starts out with trivial patches, and if a long time contributor who will complain loudly on the internet (like I am here) if his trivial patches aren’t merged can’t get it in, what chance on earth does a newcomer have?
    In case you’re wondering, this is the patch:

    --- a/sql/rpl_rli_pdb.cc
    +++ b/sql/rpl_rli_pdb.cc
    @@ -470,7 +470,7 @@ bool Slave_worker::read_info(Rpl_info_handler *from)
     
     bool Slave_worker::write_info(Rpl_info_handler *to)
     {
    -  DBUG_ENTER("Master_info::write_info");
    +  DBUG_ENTER("Slave_worker::write_info");
  • InnoDB table flags in bitfield is non-optimal (bug #74831)
    With a patch since I filed this back in November 2014, it’s managed to sit idle long enough for GCC 4.8 to practically disappear from anywhere I care about, and 4.9 makes better optimization decisions. There are other reasons why C bitfields are an awful idea too.

Actually complex issues:

  • InnoDB mutex spin loop is missing GCC barrier (bug #72755)
    Again, another bug filed back in May 2014, where InnoDB is doing a rather weird trick to attempt to get the compiler to not optimize away a spinloop. There’s a known good way of doing this, it’s called a compiler barrier. I’ve had a patch for nearly two years, not merged :(
  • buf_block_align relies on random timeouts, volatile rather than memory barriers (bug #74775)
    This bug was first filed in November 2014 and deals with a couple of incorrect assumptions about memory ordering and what volatile means.
    While this may only exhibit a problem on ARM and POWER processors (as well as any other relaxed memory ordering architectures, x86 is the notable exception), it’s clearly incorrect and very non-portable.
    Don’t expect MySQL 5.7 to work properly on ARM (or POWER). Try this:

    ./mysql-test-run.pl rpl.rpl_checksum_cache --repeat=10

    You’ll likely find MySQL > 5.7.5 still explodes.
    In fact, there’s also Bug #79378 which Alexey Kopytov filed with patch that’s been sitting idle since November 2015 which is likely related to this bug.

Not covered here: universal CRC32 hardware acceleration (rather than just for innodb data pages) and other locking issues (some only recently discovered). I also didn’t go into anything filed in December 2015… although in any other project I’d expect something filed in December 2015 to have been looked at by now.

Like it or not, MySQL is still upstream for all the MySQL derivatives active today. Maybe this will change as RocksDB and TokuDB gain users and if WebScaleSQL, MariaDB and Percona can foster a better development community.

POWER8 Accelerated CRC32 merged in MariaDB 10.1

Earlier on in benchmarking MySQL and MariaDB on POWER8, we noticed that on write workloads (or read workloads involving a lot of IO) we were spending a bunch of time computing InnoDB page checksums. This is a relatively well known MySQL problem and has existed for many years and Percona even added innodb_fast_checksum to Percona Server to help alleviate the problem.

In MySQL 5.6, we got the ability to use CRC32 checksums, which are great in that they’re a lot faster to compute than tho old InnoDB “new” checksum. There’s code inside InnoDB to use the x86 SSE2 crc32q instruction to accelerate performing the checksum on compatible x86 CPUs (although oddly enough, the CRC32 checksum in the binlog does not use this acceleration).

However, on POWER, we’d end up using the software implementation of CRC32, which used a lot more CPU than we’d like. Luckily, CRC32 is really common code and for POWER8, we got some handy instructions to help computing it. Unfortunately, this required brushing up on vector polynomial math in order to understand how to do it all quickly. The end result was Anton coming up with crc32-vpmsum code that we could drop into projects that embed a copy of crc32 that was about 41 times faster than the best non-vpmsum implementation.

Recently, Daniel Black took the patch that had passed through both Daniel Axten‘s and my hands and worked on upstreaming it into MariaDB and MySQL. We did some pretty solid benchmarking on the improvement you’d get, and we pretty much cannot notice the difference between innodb_checksum=off and having it use the POWER8 accelerated CRC32 checksum, which frees up maybe 30% of CPU time to be used for things like query execution! My original benchmark showed a 30% improvement in sysbench read/write workloads.

The excellent news? Two days ago, MariaDB merged POWER8 accelerated crc32! This means that IO heavy workloads on MariaDB on POWER8 will get much faster in the next release.

MySQL bug 74776 is open, with patch attached, so hopefully MySQL will merge this soon too (hint hint).

1 Million SQL Queries per second: GA MariaDB 10.1 on POWER8

A couple of days ago, MariaDB announced that MariaDB 10.1 is stable GA – around 19 months since the GA of MariaDB 10.0. With MariaDB 10.1 comes some important scalabiity improvements, especially for POWER8 systems. On POWER, we’re a bit unique in that we’re on the higher end of CPUs, have many cores, and up to 8 threads per core (selectable at runtime: 1, 2, 4 or 8/core) – so a dual socket system can easily be a 160 thread machine.

Recently, we (being IBM) announced availability of a couple of new POWER8 machines – machines designed for Linux and cloud environments. They are very much OpenPower machines, and more info is available here: http://www.ibm.com/marketplace/cloud/commercial-computing/us/en-us

Combine these two together, with Axel Schwenke running some benchmarks and you get 1 Million SQL Queries per second with MariaDB 10.1 on POWER8.

Having worked a lot on both MySQL for POWER and the firmware that ships in the S882LC, I’m rather happy that 1 Million queries per second is beyond what it was in June 2014, which was a neat hack on MySQL 5.7 that showed the potential of MySQL on POWER8 but wasn’t yet a product. Now, you can run a GA release of MariaDB on GA POWER8 hardware designed for scale-out cloud environments and get 1 Million SQL queries/second (with fewer cores than my initial benchmark last year!)

What’s even more impressive is that this million queries per second is in a KVM guest!

MariaDB Foundation board

There seems be a bit of an exodus from the MariaDB Foundation board recently… I’m not sure exactly what to make of it all, but the current members according to https://mariadb.org/en/foundation/ are:

  • Rasmus Johansson (chair)
  • Michael “Monty” Widenius
  • Jeremy Zawodny
  • Sergei Golubchik

With Jeremy Zawodny being the only non-MariaDB Corp member.

Recently, Jeremy Cole asked some people about their membership:

I’m a little worried for the project, the idea of a foundation around it and for people I count as friends who work on MariaDB.

MariaDB 10.0 on POWER

Good news for those wanting to run MariaDB on POWER systems, the latest 10.0 bzr tree (as of a couple of weeks ago) builds and runs well!

I recently pulled the latest MariaDB 10.0 from BZR and built it on a POWER8 system in the lab to run some quick tests. The MariaDB team has done some work on getting MariaDB to run on POWER recently, a bunch of which is based off my work on MySQL on POWER.

There’s obviously still some work in progress going on, but my initial results show performance within around 10% of MySQL, so with a bit of work we will hopefully see MariaDB reach performance parity.

One interesting find was the code to account for thread memory usage uses a single atomic variable: this does not scale and does end up showing up on profiles.

I’ll comment more on the code in a future post, but it looks like we will have MariaDB being functional on POWER in an upcoming release.

MariaDB & Trademarks, and advice for your project

I want to emphasize this for those who have not spent time near trademarks: trademarks are trouble and another one of those things where no matter what, the lawyers always win. If you are starting a company or an open source project, you are going to have to spend a whole bunch of time with lawyers on trademarks or you are going to get properly, properly screwed.

MySQL AB always held the trademark for MySQL. There’s this strange thing with trademarks and free software, where while you can easily say “use and modify this code however you want” and retain copyright on it (for, say, selling your own version of it), this does not translate too well to trademarks as there’s a whole “if you don’t defend it, you lose it” thing.

The law, is, in effect, telling you that at some point you have to be an arsehole to not lose your trademark. (You can be various degrees of arsehole about it when you have to, and whenever you do, you should assume that people are acting in good faith and just have not spent the last 40,000 years of their life talking to trademark lawyers like you have).Basically, you get to spend time telling people that they have to rename their product from “MySQL Headbut” to “Headbut for MySQL” and that this is, in fact, a really important difference.

You also, at some point, get to spend a lot of time talking about when the modifications made by a Linux distribution to package your software constitute sufficient changes that it shouldn’t be using your trademark (basically so that you’re never stuck if some arse comes along, forks it, makes it awful and keeps using your name, to the detriment of your project and business).

If you’re wondering why Firefox isn’t called Firefox in Debian, you can read the Mozilla trademark policy and probably some giant thread on debian-legal I won’t point to.

Of course, there’s ‘ MySQL trademark policy and when I was at Percona, I spent some non-trivial amount of time attempting to ensure we had a trademark policy that would work from a legal angle, a corporate angle, and a get-our-software-into-linux-distros-happily angle.

So, back in 2010, Monty started talking about a draft MariaDB trademark policy (see also, Ubuntu trademark policy, WordPress trademark policy). If you are aiming to create a development community around an open source project, this is something you need to get right. There is a big difference between contributing to a corporate open source product and an open source project – both for individuals and corporations. If you are going to spend some of your spare time contributing to something, the motivation goes down when somebody else is going to directly profit off it (corporate project) versus a community of contributors and companies who will all profit off it (open source project). The most successful hybrid of these two is likely Ubuntu, and I am struggling to think of another (maybe Fedora?).

Linux is an open source project, RedHat Enterprise Linux is an open source product and in case it wasn’t obvious when OpenSolaris was no longer Open, OpenSolaris was an open source product (and some open source projects have sprung up around the code base, which is great to see!). When a corporation controls the destiny of the name and the entire source code and project infrastructure – it’s a product of that corporation, it’s not a community around a project.

From the start, it seemed that one of the purposes of MariaDB was to create a developer community around a database server that was compatible with MySQL, and eventually, to replace it. MySQL AB was not very good at having an external developer community, it was very much an open source product and not a an open source project (one of the downsides to hiring just about anyone who ever submitted a patch). Things struggled further at Sun and (I think) have actually gotten better for MySQL at Oracle – not perfect, I could pick holes in it all day if I wanted, but certainly better.

When we were doing Drizzle, we were really careful about making sure there was a development community. Ultimately, with Drizzle we made a different fatal error, and one that we knew had happened to another open source project and nearly killed it: all the key developers went to work for a single company. Looking back, this is easily my biggest professional regret and one day I’ll talk about it more.

Brian Aker observed (way back in 2010) that MariaDB was, essentially, just Monty Program. In 2013, I did my own analysis on the source tree of MariaDB 5.5.31 and MariaDB 10.0.3-ish to see if indeed there was a development community (tl;dr; there wasn’t, and I had the numbers to prove it).If you look back at the idea of the Open Database Alliance and the MariaDB Foundation, actually, I’m just going to quote Henrik here from his blog post about leaving MariaDB/Monty Program:

When I joined the company over a year ago I was immediately involved in drafting a project plan for the Open Database Alliance and its relation to MariaDB. We wanted to imitate the model of the Linux Foundation and Linux project, where the MariaDB project would be hosted by a non-profit organization where multiple vendors would collaborate and contribute. We wanted MariaDB to be a true community project, like most successful open source projects are – such as all other parts of the LAMP stack.

….

The reality today, confirmed to me during last week, is that:

Those in charge at Monty Program have decided to keep ownership of the MariaDB trademark, logo and mariadb.org domain, since this will make the company more valuable to investors and eventually to potential buyers.

Now, with Monty Program being sold to/merged into (I’m really not sure) SkySQL, it was SkySQL who had those things. So instead of having Monty Program being (at least in theory) one of the companies working on MariaDB and following the Hacker Business Model, you now have a single corporation with all the developers, all of the trademarks, that is, essentially a startup with VC looking to be valuable to potential buyers (whatever their motives).

Again, I’m going to just quote Henrik on the us-vs-them on community here:

Some may already have observed that the 5.2 release was not announced at all on mariadb.org, rather on the Monty Program blog. It is even intact with the “us vs them” attitude also MySQL AB had of its community, where the company is one entity and “outside community contributors” is another. This is repeated in other communication, such as the recent Recently in MariaDB newsletter.

This was, again, back in 2010.

More recently, Jeremy Cole, someone who has pumped a fair bit of personal and professional effort into MySQL and MariaDB over the past (many) years, asked what seemed to be a really simple question on the maria-discuss mailing list. Basically, “What’s going on with the MariaDB trademark? Isn’t this something that should be under the MariaDB foundation?”

The subsequent email thread was as confusing as ever and should be held up as a perfect example about what not to do. Some of us had by now, for years, smelt something fishy going on around the talk of a community project versus the reality. At the time (October 2013), Rasmus Johansson (VP of Engineering at SkySQL and Board Member of MariaDB foundation) said this:

The MariaDB Foundation and SkySQL are currently working on the trademark issue to come up with a solution on what rights to the trademark each entity should have. Expect to hear more about this in a fairly near future.

 

MariaDB has from its beginning been a very community friendly project and much of the success of MariaDB relies in that fact. SkySQL of course respects that.

(and at the same time, there were pages that were “Copyright MariaDB” which, as it was pointed out, was not an actual entity… so somebody just wasn’t paying attention). Also, just to make things even less clear about where SkySQL the corporation, Monty Program the corporation and the MariaDB Foundation all fit together, Mark Callaghan noticed this text up on mariadb.com:

The MariaDB Foundation also holds the trademark of the MariaDB server and owns mariadb.org. This ensures that the official MariaDB development tree<https://code.launchpad.net/maria> will always be open for the MariaDB developer community.

So…. there’s no actual clarity here. I can imagine attempting to get involved with MariaDB inside a corporation and spending literally weeks talking to a legal department – which thrills significantly less than standing in lines at security in an airport does.

So, if you started off as yay! MariaDB is going to be a developer community around an open source project that’s all about participation, you may have even gotten code into MariaDB at various times… and then started to notice a bit of a shift… there may have been some intent to make that happen, to correct what some saw as some of the failings of MySQL, but the reality has shown something different.

Most recently, SkySQL has renamed themselves to MariaDB. Good luck to anyone who isn’t directly involved with the legal processes around all this differentiating between MariaDB the project, MariaDB Foundation and MariaDB the company and who owns what. Urgh. This is, in no way, like the Linux Foundation and Linux.

Personally, I prefer to spend my personal time contributing to open source projects rather than products. I have spent the vast majority of my professional life closer to the corporate side of open source, some of which you could better describe as closer to the open source product end of the spectrum. I think it is completely and totally valid to produce an open source product. Making successful companies, products and a butt-ton of money from open source software is an absolutely awesome thing to do and I, personally, have benefited greatly from it.

MariaDB is a corporate open source product. It is no different to Oracle MySQL in that way. Oracle has been up front and honest about it the entire time MySQL has been part of Oracle, everybody knew where they stood (even if you sometimes didn’t like it). The whole MariaDB/Monty Program/SkySQL/MariaDB Foundation/Open Database Alliance/MariaDB Corporation thing has left me with a really bitter taste in my mouth – where the opportunity to create a foundation around a true community project with successful business based on it has been completely squandered and mismanaged.

I’d much rather deal with those who are honest and true about their intentions than those who aren’t.

My guess is that this factored heavily into Henrik’s decision to leave in 2010 and (more recently) Simon Phipps’s decision to leave in August of this year. These are two people who I both highly respect, never have enough time to hang out with and I would completely trust to do the right thing and be honest when running anything in relation to free and open source software.

Maybe WebScaleSQL will succeed here – it’s a community with a purpose and several corporate contributors. A branch rather than a fork may be the best way to do this (Percona is rather successful with their branch too).

Ghosts of MySQL Past Part 5: The Era of Acquisitions

This week I’ve been writing based on my linux.conf.au 2014 talk, which you can watch the recording of.

Also see Part 1, Part 2, Part 3 and Part 4. My feed feel off Planet MySQL for a bit so you may have missed those posts.

Now we head into the era of acquisitions… there have been a few in MySQL history, and in 2005 came the second (the first was MySQL AB acquiring Alzato for NDB). In what was to be known as “InnoDB Friday”, the makers of InnoDB – Innobase Oy – was acquired by Oracle. That very same month….

MySQL 5.0 GA. The first GA release of MySQL 5.0 is infamous. It was nowhere near ready and everybody who tried to use 5.0 in the early GA days has a story about something obvious that was broken. Basically, the majority of the new features simply didn’t work. It took many point releases before people would consider 5.0 ready.

The real measure of 5.0 quality was that it took MySQL AB over a year before we started to use it for our support database.

At the end of 2005, the Maria project was started: a project to create a transactional storage engine. This should not be confused with MariaDB, which would come years later. This is Maria, now called Aria. The basic idea was to fork MyISAM and work on adding features. In hindsight, it’s easy to see that when you have a quality problem with your main product, you should probably not take a bunch of senior engineers and have them work on a different project. IIRC there was some initial estimate of a GA by the end of 2007. It’s now eight years since the project started and there’s still no stable release.

There were other efforts to get a transactional storage engine not owned by Oracle, and in 2006 MySQL AB acquired Netfrastructure and along with it Jim Starkey and Ann Harrison came to work for MySQL AB.

Originally named JSTAR, this would become known as Falcon (probably something to do with the Swedish beer by the same name).

My lca2014 talk video: Past, Present and Future of MySQL and variants

On last Wednesday morning I gave my talk at linux.conf.au 2014. You can now view and download the recording of it here:

http://mirror.linux.org.au/linux.conf.au/2014/Wednesday/28-Past_Present_and_future_of_MySQL_and_variants_-_Stewart_Smith.mp4

(hopefully more free formats will come soon, the all volunteer AV team has been absolutely amazing getting things up this quickly).

Who is working on MariaDB 10.0?

There was some suggestion after my previous post (Who works on MariaDB and MySQL?) that I look at MariaDB 10.0 – so I have. My working was very simple, in a current MariaDB 10.0 BZR tree (somewhat beyond 10.0.3), I ran the following command:

bzr log -n0 -rtag:mariadb-10.0.0..|egrep '(author|committer): '| \
  sed -e 's/^\s*//; s/committer: //; s/author: //'| \
  sort -u|grep -iv oracle

 

MariaDB foundation/MontyProgram/SkySQL:

  1. Alexander Barkov
  2. Alexey Botchkov
  3. Daniel Bartholomew
  4. Elena Stepanova
  5. Igor Babaev
  6. Jani Tolonen
  7. knielsen
  8. Michael Widenius
  9. sanja
  10. Sergei Golubchik
  11. Sergey Petrunya
  12. Sergey Vojtovich
  13. timour
  14. Vladislav Vaintroub

Elsewhere:

  1. Kentoku SHIBA (4 commits)
  2. Lixun Peng (1 commit)
  3. Olivier Bertrand (212 commits)

From Oracle (i.e. revisions merged from Oracle MySQL):

  • 81 names (which I won’t list here as 81 is a lot)

The results are no different if you go back to the first revision that is different between MariaDB 5.5 and 10.0 (found using bzr missing). Even when grepping through the bzr log for things such as “patch by”, “contribution” or “originally” I can only find 1 or two more names as original authors for patches (about the same as I can for patches going into the Oracle tree).

Please point me to revisions (revid is best way) that come from outside contributors as then I really can update this to show that there’s a larger developer community.

The current development version of Drizzle (7.2) has just as many contributors as the MariaDB development version (10.0) – although Drizzle does have fewer commits.

Who works on MariaDB and MySQL?

Looking at the committers/authors of patches in the bzr tree for MariaDB 5.5.31.

Non Oracle Contributors:

  1. Alexander Barkov
  2. Alexey Botchkov
  3. Elena Stepanova
  4. Igor Babaev
  5. knielsen
  6. Michael Widenius
  7. sanja
  8. Sergei Golubchik
  9. Sergey Petrunya
  10. timour
  11. Vladislav Vaintroub

Oracle (as they pull Oracle changes):

  1. Aditya A
  2. Akhila Maddukuri
  3. Alexander Nozdrin
  4. Anirudh Mangipudi
  5. Annamalai Gurusami
  6. Astha Pareek
  7. Balasubramanian Kandasamy
  8. Chaithra Gopalareddy
  9. Daniel Fischer
  10. Gleb Shchepa
  11. Harin Vadodaria
  12. Hery Ramilison
  13. Igor Solodovnikov
  14. Inaam Rana
  15. Jon Olav Hauglid
  16. kevin.lewis
  17. Krunal Bauskar
  18. Marc Alff
  19. Marko Mäkelä
  20. Mattias Jonsson
  21. Murthy Narkedimilli
  22. Neeraj Bisht
  23. Nisha Gopalakrishnan
  24. Nuno Carvalho
  25. Olav Sandstaa
  26. Pedro Gomes
  27. prabakaran thirumalai
  28. Praveenkumar Hulakund
  29. Ravinder Thakur
  30. Satya Bodapati
  31. sayantan.dutta
  32. Shivji Kumar Jha
  33. Sujatha Sivakumar
  34. Sunanda Menon
  35. Sunny Bains
  36. Thayumanavar
  37. Tor Didriksen
  38. Venkata Sidagam
  39. Venkatesh Duggirala
  40. Yasufumi Kinoshita

Observations:

  1. All the non-Oracle contributors work for SkySQL (and worked for Monty Program before that)
  2. Even when you go back to MariaDB 5.5.23 I can only find evidence for a maximum of 2-3 external contributions of code to MariaDB since then.
  3. In the same time frame (5.5.23-5.5.32) I see 1 or 2 going into Oracle trees, so it’s roughly the same.
  4. If you look at the contributors from Oracle over 5.5.23 to 5.5.32 there are closer to twice as many as the 40 listed above.

Somebody please correct me if I’m wrong here… perhaps MariaDB guys are just really bad at clearly marking commits that come from elsewhere? I’ve looked for “patch.*by”, “original” and “ontributed” and only turned up the above.

Are MariaDB tests adding anything extra over Oracle MySQL tests?

I grabbed all the tests introduced in MariaDB 5.5.32 (i.e. “bzr diff -rtag:mariadb-5.5.31..mariadb-5.5.32 mysql-test/” and some foo) and threw them in their own test file. I only kept tests for crashing bugs and ignored those that required plugins (there were two or three, but nothing major). So now I have a test file that should crash MariaDB 5.5.31 and probably before. But, the question is: does this crash Percona Server or MySQL?

While it is excellent to see the MariaDB guys including tests for their crashing bugs, are these MariaDB specific or do they affect other MySQL flavours?

I built a release build of top of trunk Percona Server and ran the test against it. I got no crashes. In a debug build, I got two. One was to do with REPAIR on an ARCHIVE table and the other was “SELECT UNIX_TIMESTAMP(STR_TO_DATE(‘2020′,’%Y’));”. I found the same thing for a debug build of top of tree MySQL.

All the other tests for crashing bugs, of which there were 14 – were MariaDB specific. So, out of 16 total, only 2 applied to Percona Server and MySQL.

Refactoring Internal temporary tables (another stab at it)

A few weekends ago, I started to again look at the code in Drizzle for producing internal temporary tables. Basically, we have a few type of tables:

  • Standard
  • Temporary (from CREATE TEMPORARY TABLE)
  • Temporary (from ALTER TABLE)
  • Internal temporary (to help with query execution)

If you’re lucky enough to be creating one of the first three types, you go through an increasingly lovely pile of code that constructs a nice protobuf message about what the table should look like and hands all responsibility over to the storage engine as to how to do that. The basic idea is that Drizzle gets the heck out of the way and lets the storage engine do its thing. This code path looks rather different than what we inherited from MySQL. For a start, we actually have a StorageEngine object rather than just lumping everything into the handler (which we correctly name a Cursor). However… the final part, the internal temporary table code is a bit closer to what we inherited from MySQL. There is a good reason for that, it’s ass.

For a start, the table::Singular object is still abused by Item_sum_distinct (see the setup() method) as a tuple (a table with no actual table). This is not ideal and just throws a spanner in the works for refactoring a bunch of code.

The second big problem is that create_tmp_table() doesn’t actually use any normal API calls, instead it manually sets up the table::Singular object. This includes setting up the fields for the table::Singular object in a slightly different way depending on which bit of code called create_tmp_table().

The third big problem is that it’s not storage engine agnostic. Instead of using any existing and sensible way to go and create a temporary table by using the storage engine API it instead creates a series of MI_COLUMNDEF structures which as you may be able to guess, are MyISAM specific and internal data structures.

The forth big problem is that if we end up using HEAP (again, like MyISAM, hard coded) we don’t even call the create table method on the engine. The HEAP (or MEMORY engine as it’s now known) is magic in that it can create tables on open()!

All of these issues make it really, really hard to have another engine with the ability to handle internal temporary tables. You may recall that MariaDB does include the ability to use the Aria engine for internal temporary tables. No, they did not refactor any of this code, they just made a copy of the code and put in Aria where MyISAM was along with some #ifdef for the feature.

Over the past several years I’ve tried a few times to tease this code out and start the process of turning it into something that is palatable. Every one of those times I’ve either failed or gotten sufficiently frustrated that I’ve given up.

I now have a new strategy though. After looking at the code for a good few hours a few weekends ago, I think I have an idea of where to start…. (now just for a few more free weekends to implement it).

diffstat of MySQL 5.6 versus 5.5

Yesterday I wrote about what the diffstat between MySQL 5.5 and MariaDB 5.5 was, and previously to that, about the MariaDB code size as reported by sloccount. Let’s look at MySQL 5.6.

A naive wc based “lines of code” for MySQL 5.6 sql/ directory is ~490kLOC which contasts with MySQL 5.5 being ~375kLOC by the same measure. If we diffstat the sql/ directory like I did for MariaDB 5.5 we get:

 357 files changed, 172871 insertions(+), 67922 deletions(-)

Versus, as you remember from yesterday for MariaDB 5.5 over MySQL 5.5:

 250 files changed, 83639 insertions(+), 23090 deletions(-)

The MySQL 5.5 to 5.6 sql/ changes line up with What I found in my post MySQL modularity, are we there yet? in that the core server code for MySQL has grown by about 100,000 lines of code.

The jump from MySQL 5.5 to MariaDB 5.5 is a smaller one than jumping from MySQL 5.5 to MySQL 5.6, at least in terms of changed server code.

A judgement all on if a smaller diff is a safer jump or not will rest more with the quality of that code more than anything else. As we’ve seen previously, modularity isn’t coming to the MySQL code base any time soon.

So what about the diffstat of MariaDB compared to MySQL?

So, I’ve looked at what sloccount says on the differences between Oracle MySQL over versions of itself and the various MySQL branches around. What I haven’t looked at is the diffstat. Firstly, let’s look at MariaDB.

I’m going to look at MariaDB 5.5.29 as compared to MySQL 5.5.29, both checked out from bzr. A naive diffstat would give us:

 5261 files changed, 1086165 insertions(+), 122751 deletions(-)

And this looks like an awful lot of code that has changed: about 1,086,165 lines! This actually includes a whole other copy of InnoDB in the form of XtraDB. If we take that into account we get:

 5032 files changed, 864997 insertions(+), 125099 deletions(-)

Which is still incredibly high. Let’s look at what’s changed though. We actually see a bunch of changes in the test suite, some of which are relatively harmless, while others, like the change to rpl_tests/rpl_innodb.test have a “–replace_result MyISAM InnoDB” line added to them, which is awfully odd (possibly legitimate, but it stuck out).

In the end, I came up with this diff command which I think leaves us with a best diff for what is the code difference between MySQL 5.5 and MariaDB 5.5:

 diff -Nru --exclude=BUILD* --exclude=.bzr* --exclude debian* \
--exclude=man* --exclude=mysql-test* --exclude=win* \
--exclude=unittest* --exclude=test* \
--exclude=support-files* --exclude=README \
--exclude=Docs --exclude=CMakeLists.txt \
--exclude=COPYING.LESSER --exclude=INSTALL* \
--exclude=KNOWN_BUGS.txt \
--exclude=cmake* mysql-5.5.29/ mariadb-5.5.29/

This is not to discount the build and test changes that MariaDB have made, but in this case I feel they distort the numbers a bit and I’ve previously just been counting C and C++ code, so it’s probably fairer this way.

We end up with a diffstat of:

 1156 files changed, 326081 insertions(+), 42751 deletions(-)

If we then exclude the copyright notice changes and any whitespace by changing the start of the diff command to this:

diff -NruiEbwB --ignore-matching-lines='Copyright.*Monty' \
--ignore-matching-lines='Copyright.*Oracle'

We end up with a diffstat of:

 1129 files changed, 322821 insertions(+), 39588 deletions(-)

Which is a little different to what I found in my previous post (MariaDB code size) that just used sloccount. There we found that MariaDB 5.5 was 187,000 more lines of code than MySQL 5.5 while here we find the difference to be 283,000 lines of code. I suspect these differences to be in how diff and sloccount count things. If you do a naive count of the number of lines in source files in the sql/ directory you get 375kLOC while sloccount says 256kLOC.

There is still some noise in this number as there’s some Copyright notices for some of the strings code that changes, but this doesn’t seem to be too much. What about server code though? If we just diffstat the sql/ directory (core server code), then we get:

 250 files changed, 83639 insertions(+), 23090 deletions(-)

Which is still nothing to sneeze at, sloccount tells me that MySQL 5.5.29 only has 256kLOC in the sql/ directory to begin with and a naive wc count to be about 375kLOC.

Which is bigger: MySQL or PostgreSQL?

From my previous posts, we have some numbers (excluding NDB) for the size of MySQL, so what about PostgreSQL? Here, I used PostgreSQL git trunk and classing things in the contrib/ directory as plugins. I put the number of lines of code in the src/backend/storage directory down as storage engines LoC but did not count it as non-kernel code.

Version Total LoC Plugin LoC Storage Engines LoC Remaining (kernel)
MySQL 5.5.30 858,441 2,706 171,009 684,726 (79% kernel)
MySQL 5.6.10 1,049,344 29,122 236,067 784,155 (74% kernel)
MariaDB 5.5 1,142,118 11,781 304,015 826,322 (72% kernel)
Drizzle trunk 334,810 31,150 130,727 172,933 (51% kernel)
PostgreSQL trunk 648,691 61,934 17,802 586,757 (90% kernel)

What we can see is that the PostgreSQL kernel size is actually smaller than any recent MySQL version (5.1 was slightly smaller). This is rather interesting as it is generally thought that PostgreSQL does more than MySQL. What’s more telling is that total code size, PostgreSQL is about half of MySQL 5.6 or MariaDB 5.5. Only Drizzle ends up being smaller, which makes sense as it “does less”.

Is MySQL bigger than Linux?

I’m going to take the numbers from my previous post, MySQL Modularity, Are We There Yet? for the “kernel” size of MySQL – that is, everything that isn’t a plugin or storage engine.

For Linux kernel, I’m just going to use the a-bit-old git tree I have on my laptop. I’ve decided that the following directories are for “plugins” drivers/ arch/ sound/ firmware/ crypto/ usr/ virt/ tools/ scripts/ fs/*/* and everything else is core kernel code.

Version Total LoC Total Plugin LoC Remaining (kernel)
MySQL 5.6.10 1,049,344 265,189 784,155 (74% kernel)
MariaDB 5.5 1,142,118 315,796 826,322 (72% kernel)
Linux 9,983,269 8,824,121 1,159,148 (11% kernel)

The scary thing is that it’s surprisingly close, MySQL/MariaDB core is roughly 68-71% the  size of the Linux kernel. This is probably an unfairly large number for Linux too as there’s much more of Linux that is pluggable and modular… so I actually suspect they’re closer to exactly the same size.

If we look at the net/ directory in linux, it’s a grand total of 493,000 lines of code, all of which is fairly modular and independent. You could, quite reasonably, claim that the core of Linux is in fact closer to half a million lines of code than a million, making MySQL significantly larger.

So how many engineers are looking after each code base? We know there are over a thousand Linux kernel developers contributing to each release (e.g. https://lwn.net/Articles/395961/ for data back in 2010, and https://lwn.net/Articles/537110/ for Feb 2013).

I’m now going to fudge some things to attempt to work out how many “developers” are working on linux core code rather than drivers and arch specific things. I work out there’s probably about 20-25% of linux developers who work on things that are not drivers, filesystems or arch code. This is around 250-300 developers for each kernel release.

So… how many people have ever committed code to MySQL? This is fairly easy to find out: I simply looked at the entire bzr history, grepped out every committer and then uniqued the list (this required more than just sort -u as people used different email addresses and names). How many people have ever committed code to MySQL (i.e. their code can be found in the MySQL 5.6 bzr tree)? 312.

How many committers to MySQL 5.6 are there? 161. This is pretty amazing, that’s about half of what the total is. However, this number is misleading. For example, my name is there and the last commit to the MySQL tree from me was in 2008. You also see names such as Monty Taylor and Kristian Nielsen – all three of us not having worked for MySQL/Sun/Oracle for a great number of years. At the very least, there’s been a lot of code integration into MySQL 5.6 from many existing sources that were not previously in MySQL trunk.

MySQL modularity, are we there yet?

MySQL is now over four times the size than it was with MySQL 3.23. This has not come in the shape of plugins.

Have we improved modularity over time? I decided to take LoC count for plugins and storage engines (in the case of Drizzle, memory, myisam and innobase are storage engines and everything else comes under plugin). I’ve excluded NDB from these numbers as it is rather massive and is pretty much still a separate thing.

Version Total LoC Plugin LoC Storage Engines LoC Remaining (kernel)
MySQL 3.23.58 371,987 0 (0%) 176,276 195,711 (52% kernel)
MySQL 5.1.68 721,331 228 237,124 483,979 (67% kernel)
MySQL 5.5.30 858,441 2,706 171,009 684,726 (79% kernel)
MySQL 5.6.10 1,049,344 29,122 236,067 784,155 (74% kernel)
MariaDB 5.5 1,142,118 11,781 304,015 826,322 (72% kernel)
Drizzle trunk 334,810 31,150 130,727 172,933 (51% kernel)

I’ve used the non-plugin and non-storage engine code size to be the database “kernel” – i.e. the core of the database server.

What I find really interesting here is that yes, the amount of code that is to some degree modular has increased. The amount of code that is a MySQL plugin is still very small compared to the server size

Drizzle is 20-25% of the size of a modern MySQL or MariaDB server and for many applications does largely or exactly the same thing.

Other MySQL branch code sizes

Continuing on from my previous posts, MySQL code size over releases and MariaDB code size I’ve decided to also look into some other code branches. I’ve used the same methodology as my previous few posts: sloccount for C and C++ code only.

There are also other branches around in pretty widespread use (if only within a single company). I grabbed the Google, Facebook and Twitter patches and examined them too, along with Percona Server 5.1 and 5.5.

Codebase LoC (C, C++) +/- from MySQL
Google v4 patch 5.0.37 970,110 +26,378 (from MySQL 5.0.37)
MySQL@Facebook 1,087,715 +15,768 (from MySQL 5.1.52)
Twitter 5.5.29.t10 1,192,718 +3,624
Percona Server 5.1 trunk 1,066,418 +14,878 (from MySQL 5.1.66)
Percona Server 5.5 trunk 1,208,577 +19,483 (from MySQL 5.5.29) +142,159 (from PS 5.1)
Drizzle trunk 334,810

The Google patch has always had a reputation of being large, and with an extra 26kLOC of code, it certainly is the biggest of any of the more current branches – and that’s actually a surprise to me that it adds this much code.

The Facebook and Percona Server 5.1 branches are amazingly similar in how much extra code they add, and they’re not carbon copies of each other. The Twitter patch quite notable for how little extra code it adds.

For giggles, I included Drizzle – which is (even with all the plugins) less than a third of the size of MySQL 5.1.

It’s clear that the Percona Server and Facebook patches introduce much less code than MariaDB does, which does go with the general wisdom of them being closer to Oracle MySQL than MariaDB is.

If we look at Percona Server, we see that with Percona Server 5.5 there is indeed a bunch more code than was in Percona Server 5.1, with roughly 5,000 more lines of code than we’d expect from a simple port from MySQL 5.1 to MySQL 5.5. This feels about right, we’ve added new things to Percona Server 5.5 that weren’t in Percona Server 5.1.

MariaDB code size

Continuing on from my previous post, MySQL code size over releases.

I wanted to look at the different branches/patch sets of MySQL out there and work out how far from upstream they deviated. I’m just going to compare against whatever upstream version the most easily accessible version is based on (be it 5.0.x, 5.1.x or whatever).

For MariaDB versions, I removed innodb_plugin and replaced it with xtradb for stats purposes as the MariaDB innodb_plugin is essentially the same as upstream and I don’t want to artificially inflate the diff size.

The first three major versions of MariaDB were all based on MySQL 5.1. I used sloccount and only counted C and C++ code.

So, let’s look at some of the MySQL patch sets/branches that are around. Firstly, let’s look at MariaDB:

Codebase LoC (C, C++) +/- from MySQL +/- from prev maj Version
MariaDB 5.1 1,210,168 +157,532 0
MariaDB 5.2 1,227,434 +174,798 +17,266 (since MariaDB 5.1)
MariaDB 5.3 1,264,995 +212,359 +37,561 (since MariaDB 5.2)
MariaDB 5.5 1,377,405 +187,658 (from MySQL 5.5) +112,410 (since MariaDB 5.3)

From my previous post on lines of code in MySQL versions, we learned that with MySQL 5.6 we saw a 354kLOC increase over MySQL 5.5. What is quite surprising is how close some of the MariaDB differences are to this. With MariaDB 5.5, we’re looking at a 187kLOC difference, which is roughly two thirds that of MySQL 5.6. What’s also interesting is that each incremental MariaDB release has not added nearly as much code as the MySQL 5.1 to 5.5 and 5.5 to 5.6 jumps did.

MariaDB LoC over major versions

The MariaDB code size has also been increasing, if we look at the graph above  you can really see the jump in code size over the past few releases.

If we look at the delta between MariaDB and MySQL, the first MariaDB release (MariaDB 5.1) was certainly a large jump. Each incremental MariaDB release (5.2 and 5.3) have been a smaller delta than the initial one. With MariaDB 5.5 we actually decrease the delta from MySQL, which is something that’s interesting to look at.

If we were going a straight port of MariaDB 5.3 to be based off MySQL 5.5, we’d expect the delta to be around 137kLOC (what MySQL 5.1 to 5.5 is) but it isn’t. The difference to MariaDB 5.5 from MariaDB 5.3 is only ~112kLOC, and the on the whole delta decreases.

But what makes up this big initial jump for MariaDB? Let’s look at some of the MariaDB 5.1 only modules and what’s left:

MariaDB 5.1 component LoC (MariaDB 5.1)
PBXT 45,107
FederatedX 3,076
IBM DB2i 13,486
Total 61,669
Other 95,863

So the MariaDB delta is not increase just because they included some existing modules, there’s more code in there, about as much as any major MySQL version bump.

Tomorrow we look at other MySQL branches, and we see that the MariaDB delta truly is significantly larger than any other MySQL branch.

Is your Storage Engine buggy or the database server?

If your storage engine returns an error from rnd_init (or doStartTableScan as it’s named in Drizzle) and does not save this error and return it in any subsequent calls to rnd_next, your engine is buggy. Namely it is buggy in that a) an error may not be reported back to the user and b) everything may explode horribly when rnd_next is called after rnd_init returned an error.

Unless it is running on MariaDB 5.2 or (soon, when the patch hits the tree) Drizzle.

Monty (Widenius, not Taylor) wrote a patch for MariaDB based on my bug report that addressed that problem. It uses the compiler feature to throw a warning if the result of a function isn’t checked to make sure that all places that call rnd_init are checking for an error from the engine.

Today I (finally) pulled that into Drizzle as well.

So… if your engine does the logical thing and goes “oh look, this method returns an error… I’ll return my error” it will exhibit bugs in MySQL but not MariaDB 5.2 or Drizzle (when patch hits).

Which is buggy, the server or the engine?

The MySQL bug number is 54166, filed in June 2010.