linux-kernel - Re: [PATCH 1/1] mm/swap.c: flush lru_add pvecs on compound page arrival

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Date:	Thu, 9 Jun 2016 14:21:41 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Lukasz Odzioba <lukasz.odzioba@...el.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
	aarcange@...hat.com, vdavydov@...allels.com, mingli199x@...com,
	minchan@...nel.org, lukasz.anaczkowski@...el.com,
	"Shutemov, Kirill" <kirill.shutemov@...el.com>
Subject: Re: [PATCH 1/1] mm/swap.c: flush lru_add pvecs on compound page
 arrival

On Wed 08-06-16 09:34:01, Dave Hansen wrote:
> On 06/08/2016 09:06 AM, Michal Hocko wrote:
> >> > Do we have any statistics that tell us how many pages are sitting the
> >> > lru pvecs?  Although this helps the problem overall, don't we still have
> >> > a problem with memory being held in such an opaque place?
> > Is it really worth bothering when we are talking about 56kB per CPU
> > (after this patch)?
> 
> That was the logic why we didn't have it up until now: we didn't
> *expect* it to get large.  A code change blew it up by 512x, and we had
> no instrumentation to tell us where all the memory went.
> 
> I guess we don't have any other ways to group pages than compound pages,
> and _that_ one is covered now...

exactly and that is why I am not sure it is needed. I do not expect we
would ever change the pagevec size or have a different way of grouping
pages on the LRU list.

That being said I am not objecting to the counter, I am just not sure it
is worth it.

> for one of the 5 classes of pvecs.
> 
> Is there a good reason we don't have to touch the other 4 pagevecs, btw?

I agree it would be better to do the same for others as well. Even if
this is not an immediate problem for those.

-- 
Michal Hocko
SUSE Labs