From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Minchan Kim <minchan@kernel.org>, Mel Gorman <mgorman@suse.de>,
Michal Nazarewicz <mina86@mina86.com>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
Christoph Lameter <cl@linux.com>, Rik van Riel <riel@redhat.com>,
David Rientjes <rientjes@google.com>
Subject: Re: [PATCH 4/5] mm, compaction: always update cached scanner positions
Date: Tue, 4 Nov 2014 09:28:50 +0900 [thread overview]
Message-ID: <20141104002850.GA8412@js1304-P5Q-DELUXE> (raw)
In-Reply-To: <5453B088.6080605@suse.cz>
On Fri, Oct 31, 2014 at 04:53:44PM +0100, Vlastimil Babka wrote:
> On 10/28/2014 08:08 AM, Joonsoo Kim wrote:
> >>
> >>>And, I guess that pageblock skip feature effectively disable pageblock
> >>>rescanning if there is no freepage during rescan.
> >>
> >>If there's no freepage during rescan, then the cached free_pfn also
> >>won't be pointed to the pageblock anymore. Regardless of pageblock skip
> >>being set, there will not be second rescan. But there will still be the
> >>first rescan to determine there are no freepages.
> >
> >Yes, What I'd like to say is that these would work well. Just decreasing
> >few percent of scanning page doesn't look good to me to validate this
> >patch, because there is some facilities to reduce rescan overhead and
>
> The mechanisms have a tradeoff, while this patch didn't seem to have
> negative consequences.
>
> >compaction is fundamentally time-consuming process. Moreover, failure of
> >compaction could cause serious system crash in some cases.
>
> Relying on successful high-order allocation for not crashing is
> dangerous, success is never guaranteed. Such critical allocation
> should try harder than fail due to a single compaction attempt. With
> this argument you could aim to remove all the overhead reducing
> heuristics.
>
> >>>This patch would
> >>>eliminate effect of pageblock skip feature.
> >>
> >>I don't think so (as explained above). Also if free pages were isolated
> >>(and then returned and skipped over), the pageblock should remain
> >>without skip bit, so after scanners meet and positions reset (which
> >>doesn't go hand in hand with skip bit reset), the next round will skip
> >>over the blocks without freepages and find quickly the blocks where free
> >>pages were skipped in the previous round.
> >>
> >>>IIUC, compaction logic assume that there are many temporary failure
> >>>conditions. Retrying from others would reduce effect of this temporary
> >>>failure so implementation looks as is.
> >>
> >>The implementation of pfn caching was written at time when we did not
> >>keep isolated free pages between migration attempts in a single
> >>compaction run. And the idea of async compaction is to try with minimal
> >>effort (thus latency), and if there's a failure, try somewhere else.
> >>Making sure we don't skip anything doesn't seem productive.
> >
> >free_pfn is shared by async/sync compaction and unconditional updating
> >causes sync compaction to stop prematurely, too.
> >
> >And, if this patch makes migrate/freepage scanner meet more frequently,
> >there is one problematic scenario.
>
> OK, so you don't find a problem with how this patch changes
> migration scanner caching, just the free scanner, right?
> So how about making release_freepages() return the highest freepage
> pfn it encountered (could perhaps do without comparing individual
> pfn's, the list should be ordered so it could be just the pfn of
> first or last page in the list, but need to check that) and updating
> cached free pfn with that? That should ensure rescanning only when
> needed.
Hello,
Updating cached free pfn in release_freepages() looks good to me.
In fact, I guess that migration scanner also has similar problems, but,
it's just my guess. I admit your following arguments in patch description.
However, the downside is that potentially many pages are rescanned without
successful isolation. At worst, there might be a page where isolation from LRU
succeeds but migration fails (potentially always).
So, I'm okay if you update cached free pfn in release_freepages().
Thanks.
next prev parent reply other threads:[~2014-11-04 0:27 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-07 15:33 [PATCH 0/5] Further compaction tuning Vlastimil Babka
2014-10-07 15:33 ` [PATCH 1/5] mm, compaction: pass classzone_idx and alloc_flags to watermark checking Vlastimil Babka
2014-10-20 15:45 ` Rik van Riel
2014-10-27 6:46 ` Joonsoo Kim
2014-10-27 9:11 ` Vlastimil Babka
2014-10-28 7:16 ` Joonsoo Kim
2014-10-29 13:51 ` Vlastimil Babka
2014-10-31 7:49 ` Joonsoo Kim
2014-11-14 8:52 ` Vlastimil Babka
2014-10-07 15:33 ` [PATCH 2/5] mm, compaction: simplify deferred compaction Vlastimil Babka
2014-10-15 22:32 ` Andrew Morton
2014-10-16 15:11 ` Vlastimil Babka
2014-10-07 15:33 ` [PATCH 3/5] mm, compaction: defer only on COMPACT_COMPLETE Vlastimil Babka
2014-10-20 15:18 ` Rik van Riel
2014-10-07 15:33 ` [PATCH 4/5] mm, compaction: always update cached scanner positions Vlastimil Babka
2014-10-20 15:26 ` Rik van Riel
2014-10-27 7:35 ` Joonsoo Kim
2014-10-27 9:39 ` Vlastimil Babka
2014-10-28 7:08 ` Joonsoo Kim
2014-10-31 15:53 ` Vlastimil Babka
2014-11-04 0:28 ` Joonsoo Kim [this message]
2014-11-14 8:57 ` Vlastimil Babka
2014-10-07 15:33 ` [PATCH 5/5] mm, compaction: more focused lru and pcplists draining Vlastimil Babka
2014-10-20 15:44 ` Rik van Riel
2014-10-27 7:41 ` Joonsoo Kim
2014-11-03 8:12 ` Vlastimil Babka
2014-11-04 0:37 ` Joonsoo Kim
2014-11-13 12:47 ` Vlastimil Babka
2014-11-14 7:05 ` Joonsoo Kim
2014-11-19 22:53 ` Vlastimil Babka
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20141104002850.GA8412@js1304-P5Q-DELUXE \
--to=iamjoonsoo.kim@lge.com \
--cc=akpm@linux-foundation.org \
--cc=cl@linux.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mgorman@suse.de \
--cc=mina86@mina86.com \
--cc=minchan@kernel.org \
--cc=n-horiguchi@ah.jp.nec.com \
--cc=riel@redhat.com \
--cc=rientjes@google.com \
--cc=vbabka@suse.cz \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).