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

[Mel Gorman]

On Wed, Feb 17, 2010 at 10:55:08AM +0100, 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?

It's possible but the window is small. When a threshold is reached on the
PCP lists, they get drained to the buddy lists and later picked up again
by __rmqueue_fallback(). I had considered the possibility of pages of the
wrong type being on the PCP lists which led to the patch "page-allocator:
Fallback to other per-cpu lists when the target list is empty and memory is
low" but you reported it made no difference even when fallback was allowed
with high watermarks.

> 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 -

Unless a large number of pages are pinned on those PCP lists (which
doesn't appear to be the case or the fallback patch would have helped),
the pages of other migratetypes are found by __rmqueue_fallback().

> btw here comes patch e084b.
>

Which still has zero explanation.

> 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?
>

They get found by __rmqueue_fallback().

> 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.
>
>> 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!

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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