Re: poor domU VBD performance.

[Jens Axboe <axboe@suse.de>]

On Thu, Mar 31 2005, Keir Fraser wrote:
> 
> On 31 Mar 2005, at 21:49, Andrew Theurer wrote:
> 
> >BTW, I am now getting this with xen-unstable:
> >
> >Process xenblkd (pid: 730, threadinfo=f7cc4000 task=f7c42510)
> >Stack: c022d172 f44b1a08 f363c6f0 f7cc4000 c046d40c c02849f8 f44b1a08 
> >00000010
> >       00000000 f7c42510 c0115b0a 00000000 00000000 f7c42510 c17f1e48 
> >c01092e6
> >       00000000 f7c42510 c0115b0a 00100100 00200200 00000000 00000000 
> >00000000
> 
> I wonder if blk_run_queue() is not the right thing to call. For 
> example, it ignores whether the queue has been forcibly stopped by the 
> underlying driver and doesn't check whether there are any requests that 
> actually require pushing. Plus various drivers (swraid and probably 
> lvm) have their own unplug function and blk_run_queue doesn't handle 
> that.
> 
> Could you try again, but replace calls to blk_run_queue(plugged_queue) 
> in blkback.c with:
>    if ( plugged_queue->unplug_fn )
>           plugged_queue->unplug_fn(plugged_queue);
> 
> This looks like a better match with what various other drivers do (e.g. 
> swraid).

Yes you are right, you really want to just unplug it. That should work
correctly in all cases. Remember that ->unplug_fn must not be called
with any locks called.

-- 
Jens Axboe