On 02/04/2018 09:31 PM, 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%(43% on allocation path and 38% 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.

One way to avoid this overhead is not do any merging at all for order-0
pages and leave the need for high order pages to compaction.

I think the RFC here is: we *know* this hurts high-order allocations and
Aaron demonstrated that it does make the latency worse.  But,
unexpectedly, it didn't totally crater them.

So, is the harm to large allocations worth the performance benefit
afforded to smaller ones by this patch?  How would we make a decision on
something like that?

If nothing else, this would make a nice companion topic to Daniel
Jordan's "lru_lock scalability" proposal for LSF/MM.

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