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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(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.

- How memory.stat changes ?
- What happens when you do test with 4M alloc/free ?

Thanks,
-Kame








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