linux-kernel - Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK

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Date:	Fri, 22 Apr 2011 10:08:59 -0700
From:	Sunil Mushran <sunil.mushran@...cle.com>
To:	Eric Blake <eblake@...hat.com>
CC:	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Christoph Hellwig <hch@...radead.org>,
	Josef Bacik <josef@...icpanda.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags

On 04/22/2011 10:03 AM, Eric Blake wrote:
>> cp can read whatever blocksize it chooses. If that block contains
>> zero, it would signal cp that maybe it should SEEK_DATA and skip
>> reading all those blocks. That's all. We are not trying to achieve
>> perfection. We are just trying to reduce cpu waste.
>>
>> If the fs supports SEEK_*, then great. If it does not, then it is no
>> worse than before.
> But providing just SEEK_DATA _is_ worse than before if you don't provide
> the correct SEEK_HOLE everywhere.  Because then your algorithm of trying
> lseek(SEEK_DATA) after every run of zeros in the hopes of an
> optimization is a wasted syscall, since it will just return your current
> offset every time, so you end up with more syscalls than if you had used
> the single lseek(SEEK_DATA) that returns the end of the file up front,
> and known that the remainder of the file has no holes to even try
> seeking past.

You are over-optimizing. strace any process on your box and you
will find numerous wasted syscalls.

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