On Sun, Nov 30, 2014 at 07:57:12PM -0800, Shaohua Li wrote: > 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?
Changing rw_is_sync() sounds risky, as it will change behavior of other parts, like CFQ. REQ_META/REQ_PRIO isn't an option, metadata does readahead too. And nobody uses REQ_AHEAD. explictly checking REQ_SYNC in blk_sq_make_request() sounds better, which is just for pluging and we use it for ages in blk_queue_bio(). -Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/