Jens Axboe <axboe@xxxxxxxxx> writes: > On 21/06/10 20.52, Jeff Moyer wrote: >> Jens Axboe <axboe@xxxxxxxxx> writes: >> >>> On 2010-06-21 11:48, Christoph Hellwig wrote: >>>> Now how do we use these flags in the block layer? >>>> >>>> - REQ_META >>>> >>>> The only place where we ever use this flag is inside the >>>> cfq scheduler. In cfq_choose_req we use it to give a meta >>>> request priority over one that doesn't have it. But before >>>> that we already do the same preference check with rw_is_sync, >>>> which evaluates to true for requests with that are either >>>> reads or have REQ_SYNC set. So for reads the REQ_META flag >>>> here effectively is a no-op, and for writes it gives less >>>> priority than REQ_SYNC. >>>> In addition to that we use it to account for pending metadata >>>> requests in cfq_rq_enqueued/cfq_remove_request which gets >>>> checked in cfq_should_preempt to give priority to a meta >>>> request if the other queue doesn't have any pending meta >>>> requests. But again this priority comes after a similar >>>> check for sync requests that checks if the other queue has >>>> been marked to have sync requests pending. >>> >>> It's also annotation for blktrace, so you can tell which parts of the IO >>> is meta data etc. The scheduler impact is questionable, I doubt it makes >>> a whole lot of difference. >> >> Really? Even after I showed the performance impact of setting that bit >> for journal I/O? >> >> http://lkml.org/lkml/2010/4/1/344 > > It's definitely a win in some cases, as you showed there as well. > My initial testing a long time ago had some nice benefits too. So > perhaps the above wasn't worded very well, I always worry that we > have regressions doing boosts for things like that. But given that > meta data is something that needs to be done before we get to the > real data, bumping priority generally seems like a good thing to do. Oh, I'm not arguing for that approach. I just wanted to make it clear that it can and does have a noticible impact. Cheers, Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
