Re: [PATCH 2/2] free_pcppages_bulk: prefetch buddy while not holding lock

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, Jan 24, 2018 at 08:57:43AM -0800, Dave Hansen wrote:
> On 01/24/2018 08:43 AM, Mel Gorman wrote:
> > I'm less convinced by this for a microbenchmark. Prefetch has not been a
> > universal win in the past and we cannot be sure that it's a good idea on
> > all architectures or doesn't have other side-effects such as consuming
> > memory bandwidth for data we don't need or evicting cache hot data for
> > buddy information that is not used.
> 
> I had the same reaction.
> 
> But, I think this case is special.  We *always* do buddy merging (well,
> before the next patch in the series is applied) and check an order-0
> page's buddy to try to merge it when it goes into the main allocator.
> So, the cacheline will always come in.
> 
> IOW, I don't think this has the same downsides normally associated with
> prefetch() since the data is always used.

That doesn't side-step the calculations are done twice in the
free_pcppages_bulk path and there is no guarantee that one prefetch
in the list of pages being freed will not evict a previous prefetch
due to collisions. At least on the machine I'm writing this from, the
prefetches necessary for a standard drain are 1/16th of the L1D cache so
some collisions/evictions are possible. We're doing definite work in one
path on the chance it'll still be cache-resident when it's recalculated.
I suspect that only a microbenchmark that is doing very large amounts of
frees (or a large munmap or exit) will notice and the costs of a large
munmap/exit are so high that the prefetch will be negligible savings.

-- 
Mel Gorman
SUSE Labs

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