Date:	Tue, 16 Feb 2010 17:47:26 +0100
From:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
To:	Mel Gorman <mel@....ul.ie>
CC:	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	epasch@...ibm.com, SCHILLIG@...ibm.com,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	christof.schmitt@...ibm.com, thoss@...ibm.com, hare@...e.de,
	gregkh@...ell.com
Subject: Re: Performance regression in scsi sequential throughput (iozone)
 due to "e084b - page-allocator: preserve PFN ordering when	__GFP_COLD is
 set"

Mel Gorman wrote:
> On Mon, Feb 15, 2010 at 04:46:53PM +0100, Christian Ehrhardt wrote:
[...]
>> The differences in asm are pretty much the same, as before rmqueue_bulk was already inlined the actually intended change to its parameters was negligible.
>> I wondered if it would be important if that is a constant value (-1) or if the reason was caused by that shift. So I tried:
>>
>>  23 @@ -965,7 +965,7 @@
>>  24                 set_page_private(page, migratetype);
>>  25                 list = &page->lru;
>>  26         }
>>  27 -       __mod_zone_page_state(zone, NR_FREE_PAGES, -(i << order));
>>  28 +       __mod_zone_page_state(zone, NR_FREE_PAGES, -i);
>>  29         spin_unlock(&zone->lock);
>>  30         return i;
>>  31  }
>>
[...]
> It "fixes" it only by not calling direct reclaim when it should :(

yeah as we both realized -1 was not right so it was more a crazy workaround :-)

Anyway after that being a dead end again I dug even deeper into the details of direct_reclaim - I think we can agree that out of the counters we already know the race between try_to_free making progress and get_page not getting a page causing the congestion_wait is source of the issue.

So what I tried to bring some more light into all that was extending my perf counters to track a few more details in direct_reclaim.
Two patches are attached and apply after the other three already available in that thread.
The intention is
a) to track the time
 a1) spent in try_to_free_pages
 a2) consumed after try_to_free_pages until get_page_from_freelist
 a3) spent in get_pages_from_freelist
b1) after seeing that order!=0 -> drain_all_pages I wondered if that might differ even all calls look like they have zero
b2) tracking the average amount of pages freed by try_to_free_pages for fast path and slow path (progres&!page)

Naming convention (otherwise it might get confusing here)
Good case - the scenario e.g. with e084b and 5f8dcc21 reverted resulting in high throughput and a low ratio of direct_reclaim running into progress&!page
Bad case - the scenario e.g. on a clean 2.6.32
Fast path - direct reclaim calls that did not run into progress&!page
Slow path - direct reclaim calls that ran into progress&!page ending up in a long congestion_wait and therefore called "slow" path

Mini summary of what we had before in huge tables:
            fast path   slow path
GOOD CASE      ~98%       ~1-3%
BAD CASE       ~70%        ~30%
-> leading to throughput impact of e.g. 600 mb/s with 16 iozone threads (worse with even more threads)

Out of the numbers I got the following things might help us to create a new approach to a solution.
The timings showed that that the so called slow case is actually much faster passing though direct_reclaim in bad case.

GOOD CASE                                        duration
a1 Fast-avg-duration_pre_ttf_2_post_ttf            164099
a2 Fast-avg-duration_post_ttf_2_pre_get_page          459
a3 Fast-avg-duration_pre_get_page_2_post_get_page     346
a1 Slow-avg-duration_pre_ttf_2_post_ttf            127621
a2 Slow-avg-duration_post_ttf_2_pre_get_page         1957
a3 Slow-avg-duration_pre_get_page_2_post_get_page     256
BAD CASE                                         duration   deviation to good case in %
a1 Fast-avg-duration_pre_ttf_2_post_ttf            122921   -25.09%
a2 Fast-avg-duration_post_ttf_2_pre_get_page          521   13.53%
a3 Fast-avg-duration_pre_get_page_2_post_get_page     244   -29.55%
a1 Slow-avg-duration_pre_ttf_2_post_ttf            109740   -14.01%
a2 Slow-avg-duration_post_ttf_2_pre_get_page          250   -87.18%
a3 Slow-avg-duration_pre_get_page_2_post_get_page     117   -54.16%

That means that in the bad case the execution is much faster. Especially in the case that eventually runs into the slow path try_to_free is 14% faster, more important the time between try_to_free and get_pages is 87%! faster => less than a fifth and finally get_page is 54% faster, but that is probably just failing in an almost empty list which is fast^^.

As I checked order which always was zero the time is not spent in drain_all_pages and the only other thing left might be cond_resched ?!
Everything else are a few assignments so it can't be much else.
But why would e.g. not running into schedule in cond_resched cause get_pages to not find anything - I don't know and I would expect it should be the other way around - the faster you get from free to get the more pages should be left.

I thought the progress try_to_free_pages is doing might be important as well so I took numbers for that too.
>From those I see that the good case as well as the bad case has an average of 62 pages freed in fast path.
But in slow path they start to differ - while the good case that is running only seldom in that path anyway frees an average of 1.46 pages (that might be the thing causing it not getting a page eventually) in the bad case it makes a progress of avg 37 pages even in slow path.

PAGES-FREED  fast path   slow path
GOOD CASE      ~62       ~1.46
BAD CASE       ~62       ~37

Thinking of it as asking "how few pages do I have to free until I fall from fast to slow path" the kernels behave different it looks wrong but interesting.
The good case only drops to slow path (!page) in case of ~1.46 pages freed while the bad case seems to enter that much earlier with even 37 pages freed.

As order is always 0 and get_page afaik about getting just "one" page I wonder where these 37 pages disappeared especially as in bad case it is much faster getting to get_pages after freeing those ~37 pages.

Comments and ideas welcome!

-- 

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, Open Virtualization 

View attachment "perf-count-direct-reclaim-durations.diff" of type "text/x-patch" (6199 bytes)

View attachment "perf-count-direct-reclaim-orderandprogress.diff" of type "text/x-patch" (4381 bytes)

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