Hi Kuai! On Mon 26-09-22 21:00:48, Yu Kuai wrote: > 在 2022/09/23 19:03, Jan Kara 写道: > > Hi Kuai! > > > > On Fri 23-09-22 18:23:03, Yu Kuai wrote: > > > 在 2022/09/23 18:06, Jan Kara 写道: > > > > On Fri 23-09-22 17:50:49, Yu Kuai wrote: > > > > > Hi, Christoph > > > > > > > > > > 在 2022/09/23 16:56, Christoph Hellwig 写道: > > > > > > On Thu, Sep 22, 2022 at 07:35:56PM +0800, Yu Kuai wrote: > > > > > > > wbt and bfq should work just fine if CONFIG_BFQ_GROUP_IOSCHED is disabled. > > > > > > > > > > > > Umm, wouldn't this be something decided at runtime, that is not > > > > > > if CONFIG_BFQ_GROUP_IOSCHED is enable/disable in the kernel build > > > > > > if the hierarchical cgroup based scheduling is actually used for a > > > > > > given device? > > > > > > . > > > > > > > > > > > > > > > > That's a good point, > > > > > > > > > > Before this patch wbt is simply disabled if elevator is bfq. > > > > > > > > > > With this patch, if elevator is bfq while bfq doesn't throttle > > > > > any IO yet, wbt still is disabled unnecessarily. > > > > > > > > It is not really disabled unnecessarily. Have you actually tested the > > > > performance of the combination? I did once and the results were just > > > > horrible (which is I made BFQ just disable wbt by default). The problem is > > > > that blk-wbt assumes certain model of underlying storage stack and hardware > > > > behavior and BFQ just does not fit in that model. For example BFQ wants to > > > > see as many requests as possible so that it can heavily reorder them, > > > > estimate think times of applications, etc. On the other hand blk-wbt > > > > assumes that if request latency gets higher, it means there is too much IO > > > > going on and we need to allow less of "lower priority" IO types to be > > > > submitted. These two go directly against one another and I was easily > > > > observing blk-wbt spiraling down to allowing only very small number of > > > > requests submitted while BFQ was idling waiting for more IO from the > > > > process that was currently scheduled. > > > > > > > > > > Thanks for your explanation, I understand that bfq and wbt should not > > > work together. > > > > > > However, I wonder if CONFIG_BFQ_GROUP_IOSCHED is disabled, or service > > > guarantee is not needed, does the above phenomenon still exist? I find > > > it hard to understand... Perhaps I need to do some test. > > > > Well, BFQ implements for example idling on sync IO queues which is one of > > the features that upsets blk-wbt. That does not depend on > > CONFIG_BFQ_GROUP_IOSCHED in any way. Also generally the idea that BFQ > > assigns storage *time slots* to different processes and IO from other > > processes is just queued at those times increases IO completion > > latency (for IOs of processes that are not currently scheduled) and this > > tends to confuse blk-wbt. > > > Hi, Jan > > Just to be curious, have you ever think about or tested wbt with > io-cost? And even more, how bfq work with io-cost? > > I haven't tested yet, but it seems to me some of them can work well > together. No, I didn't test these combinations. I actually expect there would be troubles in both cases under high IO load but you can try :) Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
