Re: [RFC v4 PATCH 2/5] mm/__free_one_page: skip merge for order-0 page unless compaction failed

[Mel Gorman]

On Wed, Oct 17, 2018 at 09:10:59PM +0800, Aaron Lu wrote:
> On Wed, Oct 17, 2018 at 11:44:27AM +0100, Mel Gorman wrote:
> > On Wed, Oct 17, 2018 at 02:33:27PM +0800, Aaron Lu wrote:
> > > Running will-it-scale/page_fault1 process mode workload on a 2 sockets
> > > Intel Skylake server showed severe lock contention of zone->lock, as
> > > high as about 80%(42% on allocation path and 35% on free path) CPU
> > > cycles are burnt spinning. With perf, the most time consuming part inside
> > > that lock on free path is cache missing on page structures, mostly on
> > > the to-be-freed page's buddy due to merging.
> > > 
> > 
> > This confuses me slightly. The commit log for d8a759b57035 ("mm,
> > page_alloc: double zone's batchsize") indicates that the contention for
> > will-it-scale moved from the zone lock to the LRU lock. This appears to
> > contradict that although the exact test case is different (page_fault_1
> > vs page_fault2). Can you clarify why commit d8a759b57035 is
> > insufficient?
> 
> commit d8a759b57035 helps zone lock scalability and while it reduced
> zone lock scalability to some extent(but not entirely eliminated it),
> the lock contention shifted to LRU lock in the meantime.
> 

I assume you meant "zone lock contention" in the second case.

> e.g. from commit d8a759b57035's changelog, with the same test case
> will-it-scale/page_fault1:
> 
> 4 sockets Skylake:
>     batch   score     change   zone_contention   lru_contention   total_contention
>      31   15345900    +0.00%       64%                 8%           72%
>      63   17992886   +17.25%       24%                45%           69%
> 
> 4 sockets Broadwell:
>     batch   score     change   zone_contention   lru_contention   total_contention
>      31   16703983    +0.00%       67%                 7%           74%
>      63   18288885    +9.49%       38%                33%           71%
> 
> 2 sockets Skylake:
>     batch   score     change   zone_contention   lru_contention   total_contention
>      31   9554867     +0.00%       66%                 3%           69%
>      63   9980145     +4.45%       62%                 4%           66%
> 
> Please note that though zone lock contention for the 4 sockets server
> reduced a lot with commit d8a759b57035, 2 sockets Skylake still suffered
> a lot from zone lock contention even after we doubled batch size.
> 

Any particuular reason why? I assume it's related to the number of zone
locks with the increase number of zones and the number of threads used
for the test.

> Also, the reduced zone lock contention will again get worse if LRU lock
> is optimized away by Daniel's work, or in cases there are no LRU in the
> picture, e.g. an in-kernel user of page allocator like Tariq Toukan
> demonstrated with netperf.
> 

Vaguely understood, I never looked at the LRU lock patches.

> > I'm wondering is this really about reducing the number of dirtied cache
> > lines due to struct page updates and less about the actual zone lock.
> 
> Hmm...if we reduce the time it takes under the zone lock, aren't we
> helping the zone lock? :-)
> 

Indirectly yes but reducing cache line dirtying is useful in itself so
they should be at least considered separately as independent
optimisations.

> > 
> > > One way to avoid this overhead is not do any merging at all for order-0
> > > pages. With this approach, the lock contention for zone->lock on free
> > > path dropped to 1.1% but allocation side still has as high as 42% lock
> > > contention. In the meantime, the dropped lock contention on free side
> > > doesn't translate to performance increase, instead, it's consumed by
> > > increased lock contention of the per node lru_lock(rose from 5% to 37%)
> > > and the final performance slightly dropped about 1%.
> > > 
> > 
> > Although this implies it's really about contention.
> > 
> > > Though performance dropped a little, it almost eliminated zone lock
> > > contention on free path and it is the foundation for the next patch
> > > that eliminates zone lock contention for allocation path.
> > > 
> > 
> > Can you clarify whether THP was enabled or not? As this is order-0 focused,
> > it would imply the series should have minimal impact due to limited merging.
> 
> Sorry about this, I should have mentioned THP is not used here.
> 

That's important to know. It does reduce the utility of the patch
somewhat but not all arches support THP and THP is not always enabled on
x86.

> > compaction. Lazy merging doesn't say anything about the mobility of
> > buddy pages that are still allocated.
> 
> True.
> I was thinking if compactions isn't enabled, we probably shouldn't
> enable this lazy buddy merging feature as it would make high order
> allocation success rate dropping a lot.
> 

It probably is lower as reclaim is not that aggressive. Add a comment
with an explanation as to why it's compaction-specific.

> I probably should have mentioned clearly somewhere in the changelog that
> the function of merging those unmerged order0 pages are embedded in
> compaction code, in function isolate_migratepages_block() when isolate
> candidates are scanned.
> 

Yes, but note that the concept is still problematic.
isolate_migratepages_block is not guaranteed to find a pageblock with
unmerged buddies in it. If there are pageblocks towards the end of the
zone with unmerged pages, they may never be found. This will be very hard
to detect at runtime because it's heavily dependant on the exact state
of the system.


> > 
> > When lazy buddy merging was last examined years ago, a consequence was
> > that high-order allocation success rates were reduced. I see you do the
> 
> I tried mmtests/stress-highalloc on one desktop and didn't see
> high-order allocation success rate dropping as shown in patch0's
> changelog. But it could be that I didn't test enough machines or using
> other test cases? Any suggestions on how to uncover this problem?
> 

stress-highalloc is nowhere near as useful as it used to be
unfortunately. It was built at a time when 4G machines were unusual.
config-global-dhp__workload_thpscale can be sometimes useful but it's
variable. There is not a good modern example of detecting allocation success
rates of highly fragmented systems at the moment which is a real pity.

> > merging when compaction has been recently considered but I don't see how
> > that is sufficient. If a high-order allocation fails, there is no
> > guarantee that compaction will find those unmerged buddies. There is
> 
> Any unmerged buddies will have page->buddy_merge_skipped set and during
> compaction, when isolate_migratepages_block() iterates pages to find
> isolate candidates, it will find these unmerged pages and will do_merge()
> for them. Suppose an order-9 pageblock, every page is merge_skipped
> order-0 page; after isolate_migratepages_block() iterates them one by one
> and calls do_merge() for them one by one, higher order page will be
> formed during this process and after the last unmerged order0 page goes
> through do_merge(), an order-9 buddy page will be formed.
> 

Again, as compaction is not guaranteed to find the pageblocks, it would
be important to consider whether a) that matters or b) find an
alternative way of keeping unmerged buddies on separate lists so they
can be quickly discovered when a high-order allocation fails.

> > also no guarantee that a page free will find them. So, in the event of a
> > high-order allocation failure, what finds all those unmerged buddies and
> > puts them together to see if the allocation would succeed without
> > reclaim/compaction/etc.
> 
> compaction is needed to form a high-order page after high-order
> allocation failed, I think this is also true for vanilla kernel?

It's needed to form them efficiently but excessive reclaim or writing 3
to drop_caches can also do it. Be careful of tying lazy buddy too
closely to compaction.

-- 
Mel Gorman
SUSE Labs