Re: get_vmalloc_info() and /proc/meminfo insanely expensive

[Andrew Morton]

On Wed, 12 Aug 2015 20:29:34 -0700 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> I just did some profiling of a simple "make test" in the git repo, and
> was surprised by the top kernel offender: get_vmalloc_info() showed up
> at roughly 4% cpu use.
> 
> It turns out that bash ends up reading /proc/meminfo on every single
> activation, and "make test" is basically just running a huge
> collection of shell scripts. You can verify by just doing
> 
>     strace -o trace sh -c "echo"
> 
> to see what bash does on your system. I suspect it's actually glibc,
> because a quick google finds the function "get_phys_pages()" that just
> looks at the "MemTotal" line (or possibly get_avphys_pageslooks at the
> MemFree" line).

And bash surely isn't interested in vmalloc stats.  Putting all these
things in the same file wasn't the smartest thing we've ever done.

> Ok, so bash is insane for caring so deeply that it does this
> regardless of anything else. But what else is new - user space does
> odd things. It's like a truism.
> 
> My gut feel for this is that we should just rate-limit this and cache
> the vmalloc information for a fraction of a second or something. Maybe
> we could expose total memory sizes in some more efficient format, but
> it's not like existing binaries will magically de-crapify themselves,
> so just speeding up meminfo sounds like a good thing.
> 
> Maybe we could even cache the whole seqfile buffer - Al? How painful
> would something like that be? Although from the profiles, it's really
> just the vmalloc info gathering that shows up as actually wasting CPU
> cycles..
> 

Do your /proc/meminfo vmalloc numbers actually change during that build?
Mine don't.  Perhaps we can cache the most recent vmalloc_info and
invalidate that cache whenever someone does a vmalloc/vfree/etc.
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