linux-kernel - Re: [PATCH] mm/vmscan: try to optimize branch procedures.

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Message-ID: <20171128093108.btuna7xp4yzkziuj@dhcp22.suse.cz>
Date:   Tue, 28 Nov 2017 10:31:08 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     jiang.biao2@....com.cn
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org,
        hillf.zj@...baba-inc.com, minchan@...nel.org, ying.huang@...el.com,
        mgorman@...hsingularity.net, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, zhong.weidong@....com.cn
Subject: Re: [PATCH] mm/vmscan: try to optimize branch procedures.

On Tue 28-11-17 17:19:10, jiang.biao2@....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 
> perspective。

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