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SubjectRe: Performance regression in scsi sequential throughput (iozone) due to "e084b - page-allocator: preserve PFN ordering when __GFP_COLD is set"


Christian Ehrhardt wrote:
> Christian Ehrhardt wrote:
>> 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.
>
> THe reason here is probably the the fact that in the bad case a lot of
> processes are waiting on congestion_wait and are therefore not runnnable
> and that way not scheduled via cond_resched.
>
> I'll test this theory today or tomorrow with cond_resched in
> direct_reclaim commented out and expect almost no difference.
>
>> 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
>
> 5f8dcc21 introduced per migrate type pcp lists, is it possible that we
> run in a scenario where try_to_free frees a lot of pages via, but of the
> wrong migrate type?
> And afterwards get_page
> At least try_to_free_page and it's called shrink_ functions is not
> migrate type aware while get_page and its subsequent buffered_rmqueue
> and rmqueue_bulk are - btw here comes patch e084b.
>
> I only see buffered_rmqueue chose a specific pcp list based on migrate
> type, and a fallback to migrate_reserve - is that enough fallback, what
> if the reserve is empty too but a few other types would not and those
> other types are the ones filled by try_to_free?

I just saw the full type iteration in __rmqueue_fallback, but still
somewhere the average of 37 freed pages need to go so that get_page
doesn't get one.

> I'll try to get a per migrate type #pages statistic after direct_reclaim
> reaches !page - maybe that can confirm some parts of my theory.

This might be still worth a try.

>> 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
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