Re: [RFC]block: don't mark flush request as SOFTBARRIER

[Shaohua Li <shaohua.li@intel.com>]

On Tue, 2011-09-13 at 15:46 +0800, Tejun Heo wrote:
> Hello, Shaohua.
> 
> On Tue, Sep 13, 2011 at 09:04:01AM +0800, Shaohua Li wrote:
> > we do the flush first and then dispatch the data. The flush is
> > already delayed a lot, so if latency is a problem, we already saw
> > it.
> 
> I don't necessarily agree with the above.  We don't induce any extra
> latency for flushes which don't overlap.
> 
> > Why not just remove SOFTBARRIER and use elv_dispatch_sort() for
> > flush data so drive can better arrange requests?
> 
> Maybe, I don't know.  Elevator sorting on writes is likely to be much
> less important to begin with.  Combined with the fact that sorting
> flush data would require more writes to be flushed by the following
> flush, I don't think it would be clear which way would be better.  If
> it can be shown that sorting flush data is better, why not?
Just found request_queue->queue_head list usually has only one entry,
because drive only takes one extra request from elevator. so either
dispatch_sort() or dispatch_add_tail() has no difference.

Thanks,
Shaohua