Hello, On Mon, Jul 04, 2016 at 10:46:54PM +0800, Jiale Li wrote: > These days, we have tested the cgroup blkio throttle function use fio, > and we found a problem that when we use buffered IO or set the big block > size like 1M, then the IO performance cannot reach the value we set. > For example we set blkio.throttle.read_bps_device as 10M, in kernel > version 4.3 IO performance can only reach 6M, and in kernel version > 4.4 the actual IO bps is only 3.1M. > > Then we did some research and find that in kernel version 4.3 brought in > blk_queue_split() function to split the big size bio into several parts, > and some of them are calling the generic_make_request() again, this result > the bio been throttled more than once. so the actual bio sent to device is > less than we expected. > > We have checked the newest kernel of 4.7-rc5, this problem is still exist. > > Based on this kind of situation, we propose a fix solution to add a flag bit > in bio to let the splited bio bypass the blk_queue_split(). Below is the patch > we used to fix this problem.
Thanks a lot for the report. Hmmm... there was another brekage around split bios, I wonder whether the two can be handled together somehow. http://lkml.kernel.org/g/1466583730-28595-1-git-send-email-lars.ellenb...@linbit.com > diff --git a/block/blk-merge.c b/block/blk-merge.c > index 2613531..7b17a65 100644 > --- a/block/blk-merge.c > +++ b/block/blk-merge.c > @@ -190,6 +190,7 @@ void blk_queue_split(struct request_queue *q, struct bio > **bio, > > bio_chain(split, *bio); > trace_block_split(q, split, (*bio)->bi_iter.bi_sector); > + bio_set_flag(*bio, BIO_SPLITED); Split's past participle form is split, so BIO_SPLIT would be right here. > generic_make_request(*bio); > *bio = split; > } > diff --git a/block/blk-throttle.c b/block/blk-throttle.c > index 47a3e54..4ffde95 100644 > --- a/block/blk-throttle.c > +++ b/block/blk-throttle.c > @@ -1403,6 +1403,10 @@ bool blk_throtl_bio(struct request_queue *q, struct > blkcg_gq *blkg, > > WARN_ON_ONCE(!rcu_read_lock_held()); > > + /* if the bio has been splited, should not throttle again */ > + if (bio_flagged(bio, BIO_SPLITED)) > + goto out; > + But that said, can't we just copy BIO_THROTLED while splitting? Thanks. -- tejun
