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Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool

From: Ian Jackson
Subject: Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool
Date: Wed, 17 Dec 2008 14:44:53 +0000

Anthony Liguori writes:
> This patch implements posix-aio using pthreads.  It immediately
> eliminates the need for fd pooling.

Well in principle I think I approve.  Having read the discussion I
think emulation of preadv/pwritev with pread/pwrite is probably fine.

However I did want to make one comment on this: I think it would be
valuable to try to settle on a good and stable design for the aio

I've spent the last several weeks tracking down a mysterious failure
of qemu-dm in our testing systems which turned out to be due to a bug
in the RHEL 4.3 glibc.  Obviously my own workstation and test box are
much newer and better than that.  Last time I merged from qemu
upstream I also spent quite a time discovering a different glibc bug.

These kind of threading/aio/etc. features are often immature in older
libcs, some of which advertise them but have subtle bugs.  If we can
at least pick an approach and an implementation, rather than
constantly changing, we'll have a chance to find and work around those

But having said that I think avoiding too much reliance on
glibc-implemented primitives (which often seem to be buggy in some
suble way) and doing it ourselves seems like a better approach.


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