linux-kernel - Re: [PATCH 1/1] fat: improve sync performance by grouping writes revised again

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Date:	Wed, 1 Nov 2006 17:17:48 -0500
From:	"Holden Karau" <holden@...scanfly.ca>
To:	"Phillip Susi" <psusi@....rr.com>
Cc:	"Jörn Engel" <joern@...nheim.fh-wedel.de>,
	"Josef Sipek" <jsipek@....cs.sunysb.edu>,
	hirofumi@...l.parknet.co.jp, linux-kernel@...r.kernel.org,
	"Holden Karau" <holdenk@...dros.com>,
	"akpm@...l.org" <akpm@...l.org>, linux-fsdevel@...r.kernel.org,
	"Nick Piggin" <nickpiggin@...oo.com.au>,
	"Matthew Wilcox" <matthew@....cx>
Subject: Re: [PATCH 1/1] fat: improve sync performance by grouping writes revised again

Hi Jörn,

If I do
c_bh = kmalloc(blah);
err= -ENOMEM;
if (!c_bh)
    goto error;
//here err = -ENOMEM
... do some stuff...
error:
return err;

It will return -ENOMEM, no? I mean I could set err back to 0 and do
something like:

c_bh = kmalloc(blah);
err= -ENOMEM;
if (!c_bh)
    goto error;
err = 0;
... do some stuf...
error:
return err;

At first glance, at least for me, I'd be scratching my head when I
looked at that.

Also given that this error state is to be an exception not the rule,
if what Phillip suggests is correct, than it would actually be a tiney
be slower. So, all in all I'd rather leave it the way it is :-)

On 11/1/06, Phillip Susi <psusi@....rr.com> wrote:
> I think this is getting into micro-optimization, which is usually bad.
> Also moving the assignment of err outside the body of the if only
> results in slightly faster code in the case where there is an error,
> since you can test and _maybe_ conditionally jump directly to the error:
> label if it is not very far away.  With the assignment in the body, the
> conditional jump must jump to the assignment followed by an
> unconditional jump to the label.
>
> In other words, the only time this micro optimization will be of benefit
> is if you are erroring out most of the time rather than only under
> exceptional conditions, AND the error label isn't too far away for a
> conditional branch to reach.  In other words, just don't do it ;)
>
> Jörn Engel wrote:
> > On Wed, 1 November 2006 13:02:12 -0500, Holden Karau wrote:
> >> On 11/1/06, Jörn Engel <joern@...nheim.fh-wedel.de> wrote:
> >>> Result would be something like:
> >>>        c_bh = kmalloc(...
> >>>        err = -ENOMEM;
> >>>        if (!c_bh)
> >>>                goto error;
> >> That wouldn't work so well since we always return err,
> >
> > I don't quite follow.  If the branch is taken, err is -ENOMEM.  If the
> > branch is not taken, err is set to 0 with the next instruction.
> >
> > Both methods definitely work.  Whether one is preferrable over the
> > other is imo 90% taste and maybe 10% better code on some architecture.
> > So just pick what you prefer.
> >
> > Jörn
> >
>
> -
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