From: Michal Hocko <mhocko@kernel.org>
To: jiang.biao2@zte.com.cn
Cc: akpm@linux-foundation.org, hannes@cmpxchg.org,
hillf.zj@alibaba-inc.com, minchan@kernel.org,
ying.huang@intel.com, mgorman@techsingularity.net,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
zhong.weidong@zte.com.cn
Subject: Re: [PATCH] mm/vmscan: try to optimize branch procedures.
Date: Tue, 28 Nov 2017 10:31:08 +0100 [thread overview]
Message-ID: <20171128093108.btuna7xp4yzkziuj@dhcp22.suse.cz> (raw)
In-Reply-To: <201711281719103258154@zte.com.cn>
On Tue 28-11-17 17:19:10, jiang.biao2@zte.com.cn wrote:
> > On Tue 28-11-17 09:49:45, Jiang Biao wrote:> > 1. Use unlikely to try to improve branch prediction. The
> > > *total_scan < 0* branch is unlikely to reach, so use unlikely.
> > >
> > > 2. Optimize *next_deferred >= scanned* condition.
> > > *next_deferred >= scanned* condition could be optimized into
> > > *next_deferred > scanned*, because when *next_deferred == scanned*,
> > > next_deferred shoud be 0, which is covered by the else branch.
> > >
> > > 3. Merge two branch blocks into one. The *next_deferred > 0* branch
> > > could be merged into *next_deferred > scanned* to simplify the code.
> >
> > How have you measured benefit of this patch?
> No accurate measurement for now.
> Theoretically, unlikely could improve branch prediction for unlikely branch.
Yes, except that this is a slow path and I suspect that branch
prediction has minimal if at all.
> It's hard to measure the benefit of 2 and 3, any idea to do that enlightened
> would be greatly appreciated. :) But it could simply code logic from coding
> perspectivea??
Well, in general I wouldn't touch the code without a clear benefit.
Theoretical but unmeasurable changes would require a bigger benefit.
I am not saying it is wrong at all but I am not conviced your patch is
really worth merging.
--
Michal Hocko
SUSE Labs
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next prev parent reply other threads:[~2017-11-28 9:31 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-28 1:49 Jiang Biao
2017-11-28 8:03 ` Michal Hocko
2017-11-28 9:19 ` jiang.biao2
2017-11-28 9:31 ` Michal Hocko [this message]
2017-11-28 9:46 ` Mel Gorman
2017-11-28 9:40 ` Mel Gorman
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