Re: [PATCH] fix IO hangs

[Jens Axboe <axboe@suse.de>]

On Fri, Sep 05 2003, Nick Piggin wrote:
> Hi, sorry for the hangs, everyone. I think I have it worked out, but
> testers and an ack from Jens would be good.
> 
> The insert_here code now does as advertised. The big difference being
> that regular blk_fs_requests will be subject to it (required for SCSI
> requeue). Unfortunately ll_rw_blk.c misuses it and will sometimes try
> to insert at requests which are not on the dispatch list, causing the
> badness.
> 
> It looks like the code was maybe used to provide an insertion hint
> for the elevator. The RB tree has now eliminated that requirement even
> if the code did work. Which it doesn't.
> 
> I can't reproduce the hangs with this patch. Please test.
> 
> 
> Aside, insert_here really seems to be quite dangerous to me. I think
> combination of barriers and an "insert at start/end" flag would be
> enough.

Please just kill insert_here, it's exceeded its life expectancy. The 2.2
io scheduler did a merge scan followed by an insertion scan, 2.3
collapsed them into one scan as an optimization. Performance oriented io
schedulers need to use better data structures.

Best would be to change it to pass a request back even for the NO_MERGE
case, if it has found a good insertion point. It's still a good idea to
be able to pass hints back like this, as it could still be a viable
optimization for _other_ io schedulers.

-- 
Jens Axboe