Re: Drastic Slowdown of 'fseek()' Calls From 2.4 to 2.6 -- VMM Change?

[Nick Piggin <nickpiggin@yahoo.com.au>]

Ingo Oeser wrote:
> On Saturday, 25. February 2006 06:16, Andrew Morton wrote:
>
> > runs like a dog on 2.6's reiserfs.  libc is doing a (probably) 128k read
> > on every fseek.
>
>
> Thats the bug. If I seek, I never like to have an read issued.
> seek should just return whether the result is a valid offset 
> in the underlying object. 
>
> It is perfectly valid to have a real time device which produces data 
> very fast and where you are allowed to skip without reading anything.
>
> This device coul be a pipe, which just allows forward seeking for exactly
> this (implemented by me some years ago).
>
>
> > - fseek is a pretty dumb function anyway - you're better off with
> >  stateless functions like pread() - half the number of syscalls, don't
> >  have to track where the file pointer is at.  I don't know if there's a
> >  pread()-like function in stdio though?
>
>
> pread and anything else not using RELATIVE descriptor offsets are not
> very useful for pipe like interfaces that can seek, but just forward.
>
> There are even cases, where you can seek forward and backward, but 
> only with relative offsets ever, because you have a circular buffer indexed by time.
> If you like to get the last N minutes, the relative index is always stable, 
> but the absolute offset jumps.
>
> So I hope glibc will fix fseek to work as advertised.
>
> But for the simple file case all your answers are valid.

Not really. The app is not silly if it does an fseek() then a _write_.
Writing page sized and aligned chunks should not require previously
uptodate pagecache, so doing a pre-read like this is a complete waste.

Actually glibc tries to turn this pre-read off if the seek is to a page
aligned offset, presumably to handle this case. However a big write
would only have to RMW the first and last partial pages, so pre-reading
128KB in this case is wrong.

And I would also say a 4K read is wrong as well, because a big read will
be less efficient due to the extra syscall and small IO.

--
SUSE Labs, Novell Inc.
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