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Re: ice-broker scan thread

From: Gavin Sherry <swm(at)linuxworld(dot)com(dot)au>
To: David Boreham <david_list(at)boreham(dot)org>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: ice-broker scan thread
Date: 2005-11-29 21:30:28
Message-ID: Pine.LNX.4.58.0511300826120.23317@linuxworld.com.au (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, 29 Nov 2005, David Boreham wrote:

>
> >By default when you use aio you get the version in libc (-lrt IIRC)
> >which has the issue I mentioned, probably because it's probably
> >optimised for the lots-of-network-connections type program where
> >multiple outstanding requests on a single fd are not meaningful. You
> >can however link in some other library which gives you kernel support.
> >However, I don't have a new enough kernel to have the kernel support so
> >I havn't tested that.
> >
> >
> Actually, after reading up on the current state of things, I'm not sure you
> can even get POSIX aio on top of kernel aio in Linux. There are also a
> few limitations in the 2.6 aio implementation that might prove troublesome:
> for example it only works with O_DIRECT.
>
> libaio gives userland access to the kernel aio api (which is different
> from POSIX aio).

Yes. The O_DIRECT issue is my biggest concern about Linux at the moment.
That being said, the plan is to only pre-fetch the next N blocks, where N
< 32, and to read them into the local buffer cache. In a situation where
space in the cache low (and prefetched pages might be pushed out before we
even get to read them), we need to provide such information to the
readahead mechanism so that it can reduce the number of blocks which it
prefetches.

Gavin

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