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[Qemu-devel] Re: [BUG] Regression of exec migration
From: |
Chris Lalancette |
Subject: |
[Qemu-devel] Re: [BUG] Regression of exec migration |
Date: |
Thu, 27 Aug 2009 15:31:12 +0200 |
User-agent: |
Thunderbird 2.0.0.21 (X11/20090320) |
Pierre Riteau wrote:
> Commit 907500095851230a480b14bc852c4e49d32cb16d makes exec migration
> much slower than before.
> I'm running the latest HEAD of qemu, on Debian Lenny 5.0.2.
>
> I'm migrating a fully booted Linux VM (also running Lenny) with 128MB
> of RAM to a file, using the following command: migrate "exec: cat >
> vmimage". The resulting file has a size of 57MB (because we save only
> what is allocated from the 128MB).
> With the current HEAD, it takes from 15 to 40 seconds (it's variable)
> to perform the migration to the file.
> With commit 907500095851230a480b14bc852c4e49d32cb16d reverted (or just
> commenting the "socket_set_nonblock(s->fd);" statement), it takes
> about 3 seconds.
>
> I did a little debugging and it appears that, when in non-blocking
> mode, a write to the popen'ed process which fails with EAGAIN (because
> cat is busy writing data on disk) will only be retried after 100 ms
> (the next timer tick?).
>
> Attached are two traces of qemu's output with DEBUG_BUFFERED_FILE
> defined.
> I modified the dprintf macro to print the result of
> qemu_get_clock(rt_clock) before the message.
> nonblock.log is with an unmodified HEAD while block.log has the
> "socket_set_nonblock(s->fd);" statement commented.
> Notice the "backend not ready, freezing" messages when nonblock is used.
Hm, interesting, thanks for the report. Let me try out a few things and get
back to you.
--
Chris Lalancette
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