Messages in this thread | | | Date | Sun, 30 Nov 2014 19:57:12 -0800 | From | Shaohua Li <> | Subject | Re: [PATCH] blk-mq: don't use rw_is_sync() to determine sync request |
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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
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