Re: memory reported in /proc/pid/status is wrongly scaled

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Aug 17 16:05, Livio Bertacco wrote:
>  Hi,
> While playing with reading process memory usage in Linux and cygwin, I
> found that cygwin reports too large values in /proc/self/status (in 2.10.0
> and earlier).
> Whenever I was allocating a few kB in my test program, the VmSize line in
> /proc/self/status was growing several times faster.
> 
> Small bash script to show the issue:
> #!/bin/bash
> pid=$$
> vmsizesplit=($(grep VmSize /proc/$pid/status))
> vmsize1="${vmsizesplit[1]}"
> echo Initial memory reported in status: $vmsize1 kB
> echo Allocating a 1000 kB string (bash can use more memory)
> eat=$(printf '%1024000s')
> vmsizesplit=($(grep VmSize /proc/$pid/status))
> vmsize2="${vmsizesplit[1]}"
> echo Current memory reported in status: $vmsize2 kB
> echo Difference is $[$vmsize2-$vmsize1] kB
> 
> Running this in cygwin on my laptop I get:
> Initial memory reported in status: 84928 kB
> Allocating a 1000 kB string (bash can use more memory)
> Current memory reported in status: 106880 kB
> Difference is 21952 kB
> 
> While bash may use quite more than 1000 kb in this case, 22x times larger
> doesn't seem right.
> 
> Checking source file fhandler_process.cc, the
> function format_process_status which writes the "status" proc file
> retrieves memory usage via get_mem_values. Get_mem_values obtains that info
> from NtQueryInformationProcess/PagefileUsage which is in bytes, then it
> scales it to pages dividing by wincap.page_size:
> 1515: *vmsize = vmc.PagefileUsage / wincap.page_size ();
> 
> Then format_process_status scales it back, in theory to bytes, and shifts
> it by 10 bits in order to print it out in kB:
> 1219:  unsigned page_size = wincap.allocation_granularity ();

Looks like this is the bug.  get_mem_values returns all values
in multiple of OS page_size (4K), but format_process_status multiplies
with allocation_granularity (64K), leading to 16 times overallocation
value.  The other caller of get_mem_values, format_process_statm,
returns number of pages.  This must be expressed in multiples of
allocation_granularity since that's the virtual page_size in Cygwin.
But in case of format_process_status we're looking at KB values, so
patch 8a32c24a7bdb0, replaceing page_size with allocation_granularity,
was incorrect.

Good catch!

I'll revert patch 8a32c24a7bdb0 for 2.11.0.


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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