Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool

[Anthony Liguori <aliguori@us.ibm.com>]

Gerd Hoffmann wrote:
>   Hi,
>
>   
>> 2) it's impossible to add new interfaces and we need a vectored read/write
>> operation to properly support a zero-copy API.
>>     
>
> I'm eager to try vectored block ops for the xenbus block backend.
>
>   
>> It performs at least as well as the current posix-aio code (in some
>> circumstances, even better).
>>     
>
> Well, I see a massive slowdown when switching from sync to aio in the
> xen backend code.  I think the reason is that due to the lack of a
> vectored interface (and thus /me submitting separate aio requests for
> each iovec element) stuff gets parallelized *way* too much and disk seek
> times are killing me.
>   

Yup.  And I'm also adding vectored IO operations.  Patches for that 
should show up in the next day or so.  We experienced the same problem 
with virtio-blk FWIW.

>> My only concern here is non-Linux Unices like FreeBSD.  They have kernel support
>> for posix-aio.  Since we cannot extend those interfaces though, I think that
>> even on those platforms we should still use a thread pool.
>>     
>
> Which might change some day in the future when we manage to get iovec
> support into posix-aio specs.
>
> I think the interface should use qemu-prefixed function and struct
> names.  The we can trivially map them to a system-provided aio
> implementation without worrying about name clashes.
>   

Yes, that's what I'm going to do before committing it.

Regards,

Anthony Liguori

> cheers,
>   Gerd
>