Shaohua Li <s...@fb.com> writes: >> I like the general approach. I do wonder whether we should hold back a >> single I/O at all times, or whether we should do something similar to >> what Mike Snitzer did in the dm-mpath driver, where he keeps track of >> the end position of the last I/O (without holding the I/O back) to >> determine whether we're doing sequential I/O. With your approach, we >> will be able to get merges from the very start of a sequential I/O >> stream. With Mike's approach, you don't get merges until the second and >> third I/O. Mike's way you get potentially better latency for random I/O >> at the cost of some extra fields in a data structure. Something to >> think about. > > Sorry for the later reply, been busy these days. > > Not sure about this. If latency is sensitive, the caller really should > flush plug immediately.
Yeah, I don't have any benchmark numbers to tip the scales one way or the other. >> > @@ -1240,6 +1283,10 @@ static void blk_mq_make_request(struct >> > request_queue *q, struct bio *bio) >> > return; >> > } >> > >> > + if (use_plug && !blk_queue_nomerges(q) && >> > + blk_attempt_plug_merge(q, bio, &request_count)) >> > + return; >> > + >> >> You've just added merging for flush/fua requets. Was that intentional? >> We don't attempt them in the sq case, and we should probably be >> consistent. > > FUA/FLUSH is in the unmerge bio flag list, so the merge will not happen. > But I agree checking it is faster. Actually, after looking at the patched code again, you didn't change that behaviour at all, so you can ignore this comment. >> > @@ -1314,7 +1358,7 @@ static void blk_sq_make_request(struct request_queue >> > *q, struct bio *bio) >> > * If we have multiple hardware queues, just go directly to >> > * one of those for sync IO. >> > */ >> > - use_plug = !is_flush_fua && !is_sync; >> > + use_plug = !is_flush_fua; >> > >> > blk_queue_bounce(q, &bio); >> >> For this part I'd rather use my patch. Would you mind rebasing your >> patch on top of the sq plugging patch I submitted? > > I'm ok, your patch looks better. I'll post some plug related patches out > soon and add your patch in them if you don't mind. If you'd like to post > by yourself, you can add my review in your patch: > > Reviewed-by: Shaohua Li <s...@fb.com> Thanks, feel free to go ahead and post it with your series. >> One final question, sort of related to this patch. At the end of >> blk_mq_make_request, we have this: >> >> run_queue: >> blk_mq_run_hw_queue(data.hctx, !is_sync || is_flush_fua); >> >> So, we don't run the queue synchronously for flush/fua. Is that logic >> right? Why don't we want to push that out immediately? > > I'm not sure. My theory is delaying flush/fua can increase the chance > the flush logic merges multiple flush into one flush, please see the > logic of flush handling. But this is just my thought, might be > completely wrong. Sure. I don't see a need to change the behaviour in this patch(set), I was just curious to hear the reasoning for it. Cheers, Jeff -- 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/
