Re: readahead on directories
From: Phillip Susi
Date: Wed Apr 21 2010 - 16:14:10 EST
On 4/21/2010 4:01 PM, Jamie Lokier wrote:
> Ok, this discussion has got a bit confused. Text above refers to
> needing to asynchronously read next block in a directory, but if they
> are small then that's not important.
It is very much important since if you ready each small directory one
block at a time, it is very slow. You want to queue up reads to all of
them at once so they can be batched.
> FIEMAP suggestion is only if you think you need to issue reads for
> multiple blocks in the _same_ directory in parallel. From what you say,
> I doubt that's important.
That may be why you suggested it, but it is also exactly what
readahead() does. It also queues the read asynchronously which is what
I really want so that I can queue more reads on other directories in one
big batch.
> That was my first suggestion: threads with readdir(); I thought it had
> been rejected hence the further discussion.
Yes, it was sort of rejected, which is why I said it's just a workaround
for now until readahead() works on directories. It will produce the
desired IO pattern but at the expense of ram and cpu cycles creating a
bunch of short lived threads that go to sleep almost immediately after
being created, and exit when they wake up. readahead() would be much
more efficient.
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