Re: [PATCH] memcg: add pgfault latency histograms

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Ying Han <yinghan@google.com> [2011-05-26 14:07:49]:

> This adds histogram to capture pagefault latencies on per-memcg basis. I used
> this patch on the memcg background reclaim test, and figured there could be more
> usecases to monitor/debug application performance.
> 
> The histogram is composed 8 bucket in ns unit. The last one is infinite (inf)
> which is everything beyond the last one. To be more flexible, the buckets can
> be reset and also each bucket is configurable at runtime.
> 

inf is a bit confusing for page faults -- no? Why not call it "rest"
or something line "> 38400". BTW, why was 600 used as base?

> memory.pgfault_histogram: exports the histogram on per-memcg basis and also can
> be reset by echoing "reset". Meantime, all the buckets are writable by echoing
> the range into the API. see the example below.
> 
> /proc/sys/vm/pgfault_histogram: the global sysfs tunablecan be used to turn
> on/off recording the histogram.
>

Why not make this per memcg?
 
> Functional Test:
> Create a memcg with 10g hard_limit, running dd & allocate 8g anon page.
> Measure the anon page allocation latency.
> 
> $ mkdir /dev/cgroup/memory/B
> $ echo 10g >/dev/cgroup/memory/B/memory.limit_in_bytes
> $ echo $$ >/dev/cgroup/memory/B/tasks
> $ dd if=/dev/zero of=/export/hdc3/dd/tf0 bs=1024 count=20971520 &
> $ allocate 8g anon pages
> 
> $ echo 1 >/proc/sys/vm/pgfault_histogram
> 
> $ cat /dev/cgroup/memory/B/memory.pgfault_histogram
> pgfault latency histogram (ns):
> < 600            2051273
> < 1200           40859
> < 2400           4004
> < 4800           1605
> < 9600           170
> < 19200          82
> < 38400          6
> < inf            0
> 
> $ echo reset >/dev/cgroup/memory/B/memory.pgfault_histogram

Can't we use something like "-1" to mean reset?

-- 
	Three Cheers,
	Balbir

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