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[Xen-devel] Re: VT is comically slow

On Thu, 06 Jul 2006 17:43:50 -0800, alex wrote:

> Anthony Liguori wrote:
>> Also, and I suspect this has more to do with your performance numbers,
>> QEMU currently does disk IO via read()/write() syscalls on an fd that's
>> open()'d without O_DIRECT.  This means everything's going through the
>> page cache.
> The QEMU code that we use doesn't go through the dom0 buffer cache, we
> modified the code to use O_DIRECT.  Can't user buffer cache and
> accelerated drivers (they go right to the disk) together, it can cause
> disk corruption.  The performance numbers we get from this version of
> QEMU is still 4 to 6 times slower that native disk I/O.

Sorry, I should have been more clear.  I presume that your drivers are a
lot like the normal paravirt drivers.  This means that you're injecting
bio's into the host that point directly to the memory in the guest.

Just using O_DIRECT wouldn't be enough in QEMU.  You would also have to
have functioning DMA (which appears broken in Xen).  Proper async support
would help too.


Anthony Liguori

>> I suspect that SCSI + linux-aio would result in close to native
>> performance.  Since SCSI is already in QEMU CVS, it's not that far off.
> You might be right, however even with pipelining and async I/O, I don't
> think it is going to get close to native I/O numbers.  I guess we'll
> just have to wait and see

> Best,
> -Alex V.

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