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SubjectRe: [RFC PATCH]vhost-blk: In-kernel accelerator for virtio block device
On 08/01/2011 04:17 PM, Avi Kivity wrote:
> On 07/29/2011 06:25 PM, Sasha Levin wrote:
>> On Fri, 2011-07-29 at 20:01 +0800, Liu Yuan wrote:
>> > Looking at this long list,most are function pointers that can not be
>> > inlined, and the internal data structures used by these functions are
>> > dozons. Leave aside code complexity, this long code path would really
>> > need retrofit. As Christoph simply put, this kind of mess is inherent
>> > all over the qemu code. So I am afraid, the 'retrofit' would end
>> up to
>> > be a re-write the entire (sub)system. I have to admit that, I am
>> > inclined to the MST's vhost approach, that write a new subsystem
>> other
>> > than tedious profiling and fixing, that would possibly goes as far as
>> > actually re-writing it.
>>
>> I don't think the fix for problematic userspace is to write more kernel
>> code.
>>
>> vhost-net improved throughput and latency by several factors, allowing
>> to achieve much more than was possible at userspace alone.
>>
>> With vhost-blk we see an improvement of ~15% - which I assume by your
>> and Christoph's comments can be mostly attributed to QEMU. Merging a
>> module which won't improve performance dramatically compared to what is
>> possible to achieve in userspace (even if it would require a code
>> rewrite) sounds a bit wrong to me
>
> Agree. vhost-net works around the lack of async zero copy networking
> interface. Block I/O on the other hand does have such an interface,
> and in addition transaction rates are usually lower. All we're saving
> is the syscall overhead.
>
Personally I too agree with Sasha Levin. But vhost-blk is the first fast
prototype that is supposed to act as a code base to do further
optimisation, which I plan to utilize kernel's internal stuff like BIO
layer, that can not be accessed from user space, to maximize the
performance for raw disk based block IO.

Yuan


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