linux-kernel - RE: [PATCH V2] tipc: Use bsearch library function

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Date:   Sat, 16 Sep 2017 13:20:28 +0000
From:   Jon Maloy <jon.maloy@...csson.com>
To:     Joe Perches <joe@...ches.com>, Ying Xue <ying.xue@...driver.com>,
        "Thomas Meyer" <thomas@...3r.de>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "tipc-discussion@...ts.sourceforge.net" 
        <tipc-discussion@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCH V2] tipc: Use bsearch library function



> -----Original Message-----
> From: netdev-owner@...r.kernel.org [mailto:netdev-
> owner@...r.kernel.org] On Behalf Of Joe Perches
> Sent: Saturday, September 16, 2017 06:18
> To: Ying Xue <ying.xue@...driver.com>; Thomas Meyer
> <thomas@...3r.de>; Jon Maloy <jon.maloy@...csson.com>;
> netdev@...r.kernel.org; tipc-discussion@...ts.sourceforge.net; linux-
> kernel@...r.kernel.org; davem@...emloft.net
> Subject: Re: [PATCH V2] tipc: Use bsearch library function
> 
> On Sat, 2017-09-16 at 18:10 +0800, Ying Xue wrote:
> > On 09/16/2017 05:58 PM, Joe Perches wrote:
> > > On Sat, 2017-09-16 at 17:36 +0800, Ying Xue wrote:
> > > > On 09/16/2017 05:26 PM, Joe Perches wrote:
> > > > > On Sat, 2017-09-16 at 17:02 +0800, Ying Xue wrote:
> > > > > > On 09/16/2017 03:50 PM, Thomas Meyer wrote:
> > > > > > > Use common library function rather than explicitly coding
> > > > > > > some variant of it yourself.
> > > > > > >
> > > > > > > Signed-off-by: Thomas Meyer <thomas@...3r.de>
> > > > > >
> > > > > > Acked-by: Ying Xue <ying.xue@...driver.com>
> > > > >
> > > > > Are you sure you want to do this?
> > > > >
> > > > > Note the comment above nameseq_find_subseq
> > > > >
> > > > >  * Very time-critical, so binary searches through sub-sequence array.
> > > > >
> > > > > What impact does this change have on performance?
> > > >
> > > > Sorry, I couldn't see any essential difference between this new
> > > > implementation and the original one except that the former tries
> > > > to use the library function - bsearch() to replace the original
> > > > binary search algorithm implemented in TIPC itself. Therefore, I
> > > > don't think the change will have a big impact on performance.
> > > >
> > > > If I miss something, please let me know.
> > >
> > > Comparison via a function pointer in bsearch is slower than direct
> > > code without the function call overhead.
> > >
> >
> > Right, but probably we can tolerate the slight sacrifice here.
> 
> What part of "very time critical" have you verified and benchmarked as
> inconsequential?
> 
> Please post your results.

I agree with Joe here. This change does not simplify anything, it does not reduce the amount of code, plus that it introduce an unnecessary outline call in a place where we have every reason to let the compiler do its optimization job properly.

///jon