{"id":4726,"date":"2020-11-25T11:23:13","date_gmt":"2020-11-25T19:23:13","guid":{"rendered":"https:\/\/www.flamingspork.com\/blog\/?p=4726"},"modified":"2020-11-25T11:23:13","modified_gmt":"2020-11-25T19:23:13","slug":"why-you-should-use-nproc-and-not-grep-proc-cpuinfo","status":"publish","type":"post","link":"https:\/\/www.flamingspork.com\/blog\/2020\/11\/25\/why-you-should-use-nproc-and-not-grep-proc-cpuinfo\/","title":{"rendered":"Why you should use `nproc` and not grep \/proc\/cpuinfo"},"content":{"rendered":"\n

There’s something really quite subtle about how the nproc<\/code> utility from GNU coreutils works. If you look at the man page<\/a>, it’s even the very first sentence:<\/p>\n\n\n\n

Print the number of processing units available to the current process, which may be less than the number of online processors.<\/p><\/blockquote>\n\n\n\n

So, what does that actually mean? Well, just because the computer some code is running on has a certain number of CPUs (and here I mean “number of hardware threads”) doesn’t necessarily mean that you can spawn a process that uses that many. What’s a simple example? Containers! Did you know that when you invoke docker<\/code> to run a container, you can easily limit how much CPU the container can use? In this case, we’re looking at the --cpuset-cpus<\/code> parameter, as the --cpus <\/code>one works differently.<\/p>\n\n\n\n

$ nproc\n8\n\n$ docker run --cpuset-cpus=0-1 --rm=true -it  amazonlinux:2\nbash-4.2# nproc\n2\nbash-4.2# exit\n\n$ docker run --cpuset-cpus=0-2 --rm=true -it  amazonlinux:2\nbash-4.2# nproc\n3\n<\/code><\/pre>\n\n\n\n
As you can see, nproc<\/code> here gets the right bit of information, so if you’re wanting to do a calculation such as “Please use up to the maximum available CPUs” as a parameter to the configuration of a piece of software (such as how many threads to run), you get the right number.<\/p>\n\n\n\n
But what if you use some of the other common methods?<\/p>\n\n\n\n
$ \/usr\/bin\/lscpu -p | grep -c \"^[0-9]\"\n8\n$ grep -c 'processor' \/proc\/cpuinfo \n8\n\n$ docker run --cpuset-cpus=0-1 --rm=true -it  amazonlinux:2\nbash-4.2# yum install -y \/usr\/bin\/lscpu\n......\nbash-4.2# \/usr\/bin\/lscpu -p | grep -c \"^[0-9]\"\n8\nbash-4.2# grep -c 'processor' \/proc\/cpuinfo \n8\nbash-4.2# nproc\n2<\/code><\/pre>\n\n\n\n
In this case, if you base your number of threads off grepping lscpu<\/code> you take another dependency (on the util-linux<\/code> package), which isn’t needed. You also get the wrong answer, as you do by grepping \/proc\/cpuinfo<\/code>. So, what this will end up doing is just increase the number of context switches, possibly also adding a performance degradation.<\/strong> It’s not just in docker<\/code> containers where this could be an issue of course, you can use the same mechanism that docker<\/code> uses anywhere you want to control resources of a process.<\/p>\n\n\n\n
Another subtle thing to watch out for is differences in \/proc\/cpuinfo<\/code> content depending on CPU architecture. You may not think it’s an issue today, but who wants to needlessly debug something?<\/p>\n\n\n\n
tl;dr: for determining “how many processes to run”: use nproc<\/code>, don’t grep lscpu<\/code> or \/proc\/cpuinfo<\/code><\/strong><\/p>\n","protected":false},"excerpt":{"rendered":"
There’s something really quite subtle about how the nproc utility from GNU coreutils works. If you look at the man page, it’s even the very first sentence: Print the number of processing units available to the current process, which may … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":35,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[1],"tags":[760,761,141,759],"class_list":["post-4726","post","type-post","status-publish","format-standard","hentry","category-general","tag-cpuinfo","tag-docker","tag-linux","tag-nproc"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p5a6n8-1ee","jetpack-related-posts":[{"id":3884,"url":"https:\/\/www.flamingspork.com\/blog\/2014\/10\/14\/mysql-5-7-5-on-power-thread-priority\/","url_meta":{"origin":4726,"position":0},"title":"MySQL 5.7.5 on POWER – thread priority","author":"Stewart Smith","date":"2014-10-14","format":false,"excerpt":"Good news everyone! 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