Re: [PATCH v4 1/4] mm/pageblock: mitigation cmpxchg false sharing in pageblock flags

Mel Gorman
2020-09-03T07:25:06Z

On Thu, Sep 03, 2020 at 03:01:20PM +0800, Alex Shi wrote:
> pageblock_flags is used as long, since every pageblock_flags is just 4
> bits, 'long' size will include 8(32bit machine) or 16 pageblocks' flags,
> that flag setting has to sync in cmpxchg with 7 or 15 other pageblock
> flags. It would cause long waiting for sync.
>
> If we could change the pageblock_flags variable as char, we could use
> char size cmpxchg, which just sync up with 2 pageblock flags. it could
> relief the false sharing in cmpxchg.
>
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>

Page block types were not known to change at high frequency that would
cause a measurable performance drop. If anything, the performance hit
from pageblocks is the lookup paths which is a lot more frequent.

What was the workload you were running that altered pageblocks at a high
enough frequency for collisions to occur when updating adjacent
pageblocks?

--
Mel Gorman
SUSE Labs