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xen-devel
[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.
Regards,
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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