Re: [PATCH] blk-mq: don't use rw_is_sync() to determine sync request

[Shaohua Li <shli@kernel.org>]

On Sun, Nov 30, 2014 at 06:35:11PM -0700, Jens Axboe wrote:
> On 11/30/2014 05:01 PM, Shaohua Li wrote:
> >Buffer read is counted as sync in rw_is_sync(). If we use it,
> >blk_sq_make_request() will not do per-process plug any more.
> >
> >I haven't changed blk_mq_make_request() yet. It makes sense to dispatch
> >REQ_SYNC request immediately. But for buffer read, it's weird not to do
> >per-process plug, as buffer read doesn't need low latency.
> >blk_mq_merge_queue_io() isn't very helpful, as we don't have delay mechanism
> >there, the queue is immediately flushed, which makes the merge very
> >superficial.
> 
> A read is sync, buffered or not. A buffered read is every bit as
> latency sensitive as an O_DIRECT read. I think it'd be fine to
> modify rw_is_sync() to disregard REQ_AHEAD as sync (and ensure it's
> carried forward in the request flags, too). At least to the extent
> that we process plug and get the merging, since for streamed reads
> we'd soon be waiting on them anyway.

A quick search shows nobody uses REQ_AHEAD. For stream reads, only first several
reads are waited I suppose, later reads are read ahead. Maybe only counts
REQ_META read as sync?

Thanks,
Shaohua