Re: [PATCH 6/9] kstaled: rate limit pages scanned per second.

[Michel Lespinasse]

On Wed, Sep 28, 2011 at 1:59 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Wed, 28 Sep 2011 01:19:50 -0700
> Michel Lespinasse <walken@xxxxxxxxxx> wrote:
>> It tends to perform worse if we try making it multithreaded. What
>> happens is that the scanning threads call page_referenced() a lot, and
>> if they both try scanning pages that belong to the same file that
>> causes the mapping's i_mmap_mutex lock to bounce. Same things happens
>> if they try scanning pages that belong to the same anon VMA too.
>>
>
> Hmm. with brief thinking, if you can scan list of page tables,
> you can set young flags without any locks.
> For inode pages, you can hook page lookup, I think.

It would be possible to avoid taking rmap locks by instead scanning
all page tables, and transferring the pte young bits observed there to
the PageYoung page flag. This is a significant design change, but
would indeed work.

Just to clarify the idea, how would you go about finding all page
tables to scan ? The most straightforward approach would be iterate
over all processes and scan their address spaces, but I don't think we
can afford to hold tasklist_lock (even for reads) for so long, so we'd
have to be a bit smarter than that... I can think of a few different
ways but I'd like to know if you have something specific in mind
first.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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