Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device

[Avi Kivity <avi@redhat.com>]

On 01/11/2010 04:37 PM, Anthony Liguori wrote:
>> That has the downside of bouncing a cache line on unrelated exits.
>
>
> The read and write sides of the ring are widely separated in physical 
> memory specifically to avoid cache line bouncing.

I meant, exits on random vcpus will cause the cacheline containing the 
notification disable flag to bounce around.  As it is, we read it on the 
vcpu that owns the queue and write it on that vcpu or the I/O thread.

>>   It probably doesn't matter with qemu as it is now, since it will 
>> bounce qemu_mutex, but it will hurt with large guests (especially if 
>> they have many rings).
>>
>> IMO we should get things to work well without riding on unrelated 
>> exits, especially as we're trying to reduce those exits.
>
> A block I/O request can potentially be very, very long lived.  By 
> serializing requests like this, there's a high likelihood that it's 
> going to kill performance with anything capable of processing multiple 
> requests.

Right, that's why I suggested having a queue depth at which disabling 
notification kicks in.  The patch hardcodes this depth to 1, unpatched 
qemu is infinite, a good value is probably spindle count + VAT.

> OTOH, if we aggressively poll the ring when we have an opportunity to, 
> there's very little down side to that and it addresses the 
> serialization problem.

But we can't guarantee that we'll get those opportunities, so it doesn't 
address the problem in a general way.  A guest that doesn't use hpet and 
only has a single virtio-blk device will not have any reason to exit to 
qemu.

-- 
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