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Date: Sat, 7 Nov 2009 04:03:17 +0900 (JST)
Subject: Re: [MM] Make mm counters per cpu instead of atomic V2
From: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>
To: "Christoph Lameter" <cl@linux-foundation.org>
Cc: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>,
       "Dave Jones" <davej@redhat.com>,
       "hugh.dickins@tiscali.co.uk" <hugh.dickins@tiscali.co.uk>,
       linux-mm@kvack.org, linux-kernel@vger.kernel.org,
       akpm@linux-foundation.org, "Tejun Heo" <tj@kernel.org>
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Christoph Lameter wrote:
> On Fri, 6 Nov 2009, KAMEZAWA Hiroyuki wrote:
>
>> BTW, can't we have single-thread-mode for this counter ?
>> Usual program's read-side will get much benefit.....
>
> Thanks for the measurements.
>
> A single thread mode would be good. Ideas on how to add that would be
> appreciated.
>

Maybe there are some ways....At brief thought....
==
struct usage_counter {
    long rss;
    long file;
}


struct mm_struct {
    ....
    atomic_long_t  rss;   /* only updated when usage_counter is NULL */
    atomic_long_t  file;  /* only updated when usage_counter is NULL */
    struct usage_counter *usage;  /* percpu counter used when
                                     multi-threaded */
    .....
}

And allocate mm->usage only when the first CLONE_THREAD is specified.

if (mm->usage)
    access per cpu
else
    atomic_long_xxx

and read operation will be

    val = atomic_read(mm->rss);
    if (mm->usage)
        for_each_possible_cpu()....
==
Does "if" seems too costly ?

If this idea is bad, I think moving mm_counter to task_struct from
mm_struct and doing slow-sync is an idea instead of percpu.

for example

struct task_struct {
    ....
    mm_counter_t temp_counter;
    ....
};

struct mm_struct {
    .....
    atomic_long_t rss;
    atomic_long_t file;
};

And adds temp_counter's value to mm_struct at some good point....before
sleep ?
kswapd and reclaim routine can update mm_struct's counter, directly.
Readers just read mm_struct's counter.

Thanks,
-Kame


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