Re: Why memory.usage_in_bytes is always increasing after every mmap/dirty/unmap sequence

[bill4carson <bill4carson@gmail.com>]

On 2012a1'03ae??23ae?JPY 16:19, KAMEZAWA Hiroyuki wrote:
> (2012/03/23 17:04), bill4carson wrote:
>
>> Hi, all
>>
>> I'm playing with memory cgroup, I'm a bit confused why
>> memory.usage in bytes is steadily increasing at 4K page pace
>> after every mmap/dirty/unmap sequence.
>>
>> On linux-3.6.34.10/linux-3.3.0-rc5
>> A simple test case does following:
>>
>> a) mmap 128k memory in private anonymous way
>> b) dirty all 128k to demand physical page
>> c) print memory.usage_in_bytes<-- increased at 4K after every loop
>> d) unmap previous 128 memory
>> e) goto a) to repeat
>
> In Documentation/cgroup/memory.txt
> ==
> 5.5 usage_in_bytes
>
> For efficiency, as other kernel components, memory cgroup uses some optimization
> to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
> method and doesn't show 'exact' value of memory(and swap) usage, it's an fuzz
> value for efficient access. (Of course, when necessary, it's synchronized.)
> If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
> value in memory.stat(see 5.2).
> ==
>
> In current implementation, memcg tries to charge resource in size of 32 pages.
> So, if you get 32 pages and free 32pages, usage_in_bytes may not change.
> This is affected by caches in other cpus and other flushing operations caused
> by some workload in other cgroups. memcg's usage_in_bytes is not precise in
> 128k degree.

Thanks for the internal design details.
I noticed on 2.6.34, it's checked on every 512 Kbytes

See http://lxr.linux.no/#linux+v2.6.34/mm/memcontrol.c#L571

And I haven't see the 3.3.0 changes.


>
> - How memory.stat changes ?


root@localhost:/sys/fs/cgroup/memory/a> cat memory.stat;cat 
memory.usage_in_bytes
cache 0
rss 131072            <------ when mmap/dirty/
mapped_file 0
pgpgin 1278
pgpgout 1246
inactive_anon 0
active_anon 131072
inactive_file 0
active_file 0
unevictable 0
hierarchical_memory_limit 9223372036854775807
total_cache 0
total_rss 131072
total_mapped_file 0
total_pgpgin 1278
total_pgpgout 1246
total_inactive_anon 0
total_active_anon 131072
total_inactive_file 0
total_active_file 0
total_unevictable 0


root@localhost:/sys/fs/cgroup/memory/a> cat memory.stat;cat 
memory.usage_in_bytes
cache 0
rss 4096            <------ when mmap/dirty/unmap
mapped_file 0
pgpgin 1278
pgpgout 1277
inactive_anon 0
active_anon 4096
inactive_file 0
active_file 0
unevictable 0
hierarchical_memory_limit 9223372036854775807
total_cache 0
total_rss 4096
total_mapped_file 0
total_pgpgin 1278
total_pgpgout 1277
total_inactive_anon 0
total_active_anon 4096
total_inactive_file 0
total_active_file 0
total_unevictable 0


> - What happens when you do test with 4M alloc/free ?
>

I tried on 2.6.34, it's the same behavior.


> Thanks,
> -Kame
>
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-- 
Love each day!

--bill

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