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[Xen-devel] Re: [Patch] Buffer disk I/O requests

Ian Pratt wrote:
How does it compare to just using the SCSI HBA support that got
checked in a few days ago (in the qemu-dm 0.9.0 upgrade)?
In our test,  the performance of SCSI HBA is better than our patch
performance in qemu 0.9.0,

Thanks for running the tests.

But we find the total I/O preformance
downgrade a lot after upgrade to qemu 0.9.0. We suspect there may be
some issues in qemu 0.9.0.

Please can you explain in more detail.

IDE emulation is largely bound by how many requests you can get our per second. In 0.8.2, DMA completion happened immediately after the IO request finished. In 0.9.0, DMA completion is triggered by a AIO completion of the event. This implies a trip through the main event loop.

By default, QEMU only allows glibc AIO to use a single thread which basically turns all AIO requests into synchronous requests. You'll get some performance back by changing that to allow multiple threads.

I haven't looked at the QEMU 0.9.0 port just yet but I suspect that's the problem since I've seen this behavior in normal QEMU.


Anthony Liguori

If we're going to add support for enabling buffering of ioport
accesses beyond what we currently special case for the VGA it should
be via a generic interface used by qemu to register sets of ports
with xen and configure how they will be handled.
Yes, if there are many these buffering cases, using a generic
is a final solution.

I'd like to see this generic mechanism introduced for more than just
whether writes are buffered or not -- it would be very useful to
register ranges of port or mmio space for handling in different
fashions, e.g.:
 * read: forward to handler domain X channel Y
 * read: read as zeros
 * write: forward to handler domain X channel Y (and flush any buffered)
 * write: buffer and forward to domain X channel Y
 * write: ignore writes
These hooks would also be very useful for adding debugging/tracing. I
severely dislike our current  approach of forwarding anything that
doesn't get picked up in Xen to a single qemu-dm rather than registering
explicit ranges.


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