Re: [PATCH 1/2]block: optimize non-queueable flush request drive

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

On Mon, 2011-04-25 at 17:13 +0800, Tejun Heo wrote:
> Hello,
> 
> On Mon, Apr 25, 2011 at 10:58:27AM +0200, Tejun Heo wrote:
> > Eh, wasn't your optimization only applicable if flush is not
> > queueable?  IIUC, what your optimization achieves is merging
> > back-to-back flushes and you're achieving that in a _very_ non-obvious
> > round-about way.  Do it in straight-forward way even if that costs
> > more lines of code.
> 
> To add a bit more, here, flush exclusivity gives you an extra ordering
> contraint that while flush is in progress no other request can proceed
> and thus if there's another flush queued, it can be completed
> together, right?  If so, teach block layer the whole thing - let block
> layer hold further requests while flush is in progress and complete
> back-to-back flushes together on completion and then resume normal
> queue processing.
the blk-flush is part of block layer. If what you mean is the libata
part, block layer doesn't know if flush is queueable without knowledge
from driver.

> Also, another interesting thing to investigate is why the two flushes
> didn't get merged in the first place.  The two flushes apparently
> didn't have any ordering requirement between them.  Why didn't they
> get merged in the first place?  If the first flush were slightly
> delayed, the second would have been issued together from the beginning
> and we wouldn't have to think about merging them afterwards.  Maybe
> what we really need is better algorithm than C1/2/3 described in the
> comment?
the sysbench fileio does a 16 threads write-fsync, so it's quite normal
a flush is running and another flush is added into pending list.

Thanks,
Shaohua