linux-kernel - Re: [HACKERS] Improve lseek scalability v3

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Message-ID: <CANGUGtDCLkKMa5vmgYtDDfh5p32aUGAtYOGFMrT6YK6oVYLBCQ@mail.gmail.com>
Date:	Tue, 20 Sep 2011 09:18:56 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Stephen Frost <sfrost@...wman.net>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Andres Freund <andres@...razel.de>,
	Andi Kleen <andi@...stfloor.org>, viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	robertmhaas@...il.com, pgsql-hackers@...tgresql.org
Subject: Re: [HACKERS] Improve lseek scalability v3

2011/9/19 Matthew Wilcox <matthew@....cx>:
> On Mon, Sep 19, 2011 at 08:31:00AM -0400, Stephen Frost wrote:
>> * Benjamin LaHaise (bcrl@...ck.org) wrote:
>> > For such tables, can't Postgres track the size of the file internally?  I'm
>> > assuming it's keeping file descriptors open on the tables it manages, in
>> > which case when it writes to a file to extend it, the internally stored size
>> > could be updated.  Not making a syscall at all would scale far better than
>> > even a modified lseek() will perform.
>>
>> We'd have to have it in shared memory and have a lock around it, it
>> wouldn't be cheap at all.
>
> Yep, that makes perfect sense.  After all, the kernel does basically the
> same thing to maintain this information; why should we have userspace
> duplicating the same infrastructure?
>
> I must admit, I'd never heard of this usage of lseek to get the current
> size of a file before; I'd assumed everybody used fstat.  Given this
> legitimate reason for a high-frequency calling of lseek, I withdraw my
> earlier objection to the patch series.
>
> --
> Matthew Wilcox                          Intel Open Source Technology Centre
> "Bill, look, we understand that you're interested in selling us this
> operating system, but compare it to ours.  We can't possibly take such
> a retrograde step."
> --
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>

I really don't understand the approach here. An improvement is an
improvement, do we need a use case to add an improvement to the
kernel? We are not talking about to add a new syscall or to do an ABI
change in this case. So my absolute ack to these patches.

Marco
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