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 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
