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SubjectRe: [PATCH v2 3/3] vmscan: implement swap token priority aging
Hi,

(2011/05/21 4:30), Andrew Morton wrote:
> On Thu, 19 May 2011 11:34:15 +0900
> KOSAKI Motohiro<kosaki.motohiro@jp.fujitsu.com> wrote:
>
>> While testing for memcg aware swap token, I observed a swap token
>> was often grabbed an intermittent running process (eg init, auditd)
>> and they never release a token.
>>
>> Why?
>>
>> Some processes (eg init, auditd, audispd) wake up when a process
>> exiting. And swap token can be get first page-in process when
>> a process exiting makes no swap token owner. Thus such above
>> intermittent running process often get a token.
>>
>> And currently, swap token priority is only decreased at page fault
>> path. Then, if the process sleep immediately after to grab swap
>> token, the swap token priority never be decreased. That's obviously
>> undesirable.
>>
>> This patch implement very poor (and lightweight) priority aging.
>> It only be affect to the above corner case and doesn't change swap
>> tendency workload performance (eg multi process qsbench load)
>>
>> ...
>>
>> --- a/mm/thrash.c
>> +++ b/mm/thrash.c
>> @@ -25,10 +25,13 @@
>>
>> #include<trace/events/vmscan.h>
>>
>> +#define TOKEN_AGING_INTERVAL (0xFF)
>
> Needs a comment describing its units and what it does, please.
> Sufficient for readers to understand why this value was chosen and what
> effect they could expect to see from changing it.

Will do.


>> static DEFINE_SPINLOCK(swap_token_lock);
>> struct mm_struct *swap_token_mm;
>> struct mem_cgroup *swap_token_memcg;
>> static unsigned int global_faults;
>> +static unsigned int last_aging;
>
> Is this a good name? Would something like prev_global_faults be better?

Current code is "next-aging = last-aging + 256global-fault". so,
prev_global_faults is slightly misleading to me.


> `global_faults' and `last_aging' could be made static local in
> grab_swap_token().

OK. even though I don't like static in a function.


>
>> void grab_swap_token(struct mm_struct *mm)
>> {
>> @@ -47,6 +50,11 @@ void grab_swap_token(struct mm_struct *mm)
>> if (!swap_token_mm)
>> goto replace_token;
>>
>> + if ((global_faults - last_aging)> TOKEN_AGING_INTERVAL) {
>> + swap_token_mm->token_priority /= 2;
>> + last_aging = global_faults;
>> + }
>
> It's really hard to reverse-engineer the design decisions from the
> implementation here, therefore... ?
>
>> if (mm == swap_token_mm) {
>> mm->token_priority += 2;
>> goto update_priority;
>> @@ -64,7 +72,7 @@ void grab_swap_token(struct mm_struct *mm)
>> goto replace_token;
>>
>> update_priority:
>> - trace_update_swap_token_priority(mm, old_prio);
>> + trace_update_swap_token_priority(mm, old_prio, swap_token_mm);
>>
>> out:
>> mm->faultstamp = global_faults;
>> @@ -80,6 +88,7 @@ replace_token:
>> trace_replace_swap_token(swap_token_mm, mm);
>> swap_token_mm = mm;
>> swap_token_memcg = memcg;
>> + last_aging = global_faults;
>> goto out;
>> }
>
> In fact all of grab_swap_token() and the thrash-detection code in
> general are pretty tricky and unobvious stuff. So we left it
> undocumented :(

Well, original swap token (designed by Rik) is pretty straight forward
implementation on the theory. Therefore we didn't need too verbose doc.
The paper give us good well documentation.

# Oh, now http://www.cs.wm.edu/~sjiang/token.pdf is dead link.
# we should fix it to http://www.cse.ohio-state.edu/hpcs/WWW/HTML/publications/papers/TR-05-1.pdf,
# maybe, or do anybody know better url?

But following commit rewrite almost all code. and we lost good documentation.
It's a bit sad.


commit 7602bdf2fd14a40dd9b104e516fdc05e1bd17952
Author: Ashwin Chaugule <ashwin.chaugule@celunite.com>
Date: Wed Dec 6 20:31:57 2006 -0800

[PATCH] new scheme to preempt swap token







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