linux-kernel - Re: [PATCH 0/2] memcg: improving scalability by reducing lock contention at charge/uncharge

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Date:	Mon, 5 Oct 2009 16:07:33 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH 0/2] memcg: improving scalability by reducing lock
 contention at charge/uncharge

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-10-02 13:55:31]:

> Hi,
> 
> This patch is against mmotm + softlimit fix patches.
> (which are now in -rc git tree.)
> 
> In the latest -rc series, the kernel avoids accessing res_counter when
> cgroup is root cgroup. This helps scalabilty when memcg is not used.
> 
> It's necessary to improve scalabilty even when memcg is used. This patch
> is for that. Previous Balbir's work shows that the biggest obstacles for
> better scalabilty is memcg's res_counter. Then, there are 2 ways.
> 
> (1) make counter scale well.
> (2) avoid accessing core counter as much as possible.
> 
> My first direction was (1). But no, there is no counter which is free
> from false sharing when it needs system-wide fine grain synchronization.
> And res_counter has several functionality...this makes (1) difficult.
> spin_lock (in slow path) around counter means tons of invalidation will
> happen even when we just access counter without modification.
> 
> This patch series is for (2). This implements charge/uncharge in bached manner.
> This coalesces access to res_counter at charge/uncharge using nature of
> access locality.
> 
> Tested for a month. And I got good reorts from Balbir and Nishimura, thanks.
> One concern is that this adds some members to the bottom of task_struct.
> Better idea is welcome.
> 
> Following is test result of continuous page-fault on my 8cpu box(x86-64).
> 
> A loop like this runs on all cpus in parallel for 60secs. 
> ==
>         while (1) {
>                 x = mmap(NULL, MEGA, PROT_READ|PROT_WRITE,
>                         MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);
> 
>                 for (off = 0; off < MEGA; off += PAGE_SIZE)
>                         x[off]=0;
>                 munmap(x, MEGA);
>         }
> ==
> please see # of page faults. I think this is good improvement.
> 
> 
> [Before]
>  Performance counter stats for './runpause.sh' (5 runs):
> 
>   474539.756944  task-clock-msecs         #      7.890 CPUs    ( +-   0.015% )
>           10284  context-switches         #      0.000 M/sec   ( +-   0.156% )
>              12  CPU-migrations           #      0.000 M/sec   ( +-   0.000% )
>        18425800  page-faults              #      0.039 M/sec   ( +-   0.107% )
>   1486296285360  cycles                   #   3132.080 M/sec   ( +-   0.029% )
>    380334406216  instructions             #      0.256 IPC     ( +-   0.058% )
>      3274206662  cache-references         #      6.900 M/sec   ( +-   0.453% )
>      1272947699  cache-misses             #      2.682 M/sec   ( +-   0.118% )
> 
>    60.147907341  seconds time elapsed   ( +-   0.010% )
> 
> [After]
>  Performance counter stats for './runpause.sh' (5 runs):
> 
>   474658.997489  task-clock-msecs         #      7.891 CPUs    ( +-   0.006% )
>           10250  context-switches         #      0.000 M/sec   ( +-   0.020% )
>              11  CPU-migrations           #      0.000 M/sec   ( +-   0.000% )
>        33177858  page-faults              #      0.070 M/sec   ( +-   0.152% )
>   1485264748476  cycles                   #   3129.120 M/sec   ( +-   0.021% )
>    409847004519  instructions             #      0.276 IPC     ( +-   0.123% )
>      3237478723  cache-references         #      6.821 M/sec   ( +-   0.574% )
>      1182572827  cache-misses             #      2.491 M/sec   ( +-   0.179% )
> 
>    60.151786309  seconds time elapsed   ( +-   0.014% )
>

I agree, I liked the previous patchset, let me re-review this one!
Definitely a good candidate to -mm. 

-- 
	Balbir
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