On Mon, Mar 05, 2012 at 11:58:01PM +0100, Andrea Righi wrote: [..] > What about this scenario? (Sorry, I've not followed some of the recent > discussions on this topic, so I'm sure I'm oversimplifying a bit or > ignoring some details): > > - track inodes per-memcg for writeback IO (provided Greg's patch) > - provide per-memcg dirty limit (global, not per-device); when this > limit is exceeded flusher threads are awekened and all tasks that > continue to generate new dirty pages inside the memcg are put to > sleep > - flusher threads start to write some dirty inodes of this memcg (using > the inode tracking feature), let say they start with a chunk of N > pages of the first dirty inode > - flusher threads can't flush in this way more than N pages / sec > (where N * PAGE_SIZE / sec is the blkcg "buffered write rate limit" > on the inode's block device); if a flusher thread exceeds this limit > it won't be blocked directly, it just stops flushing pages for this > memcg after the first chunk and it can continue to flush dirty pages > of a different memcg. > So, IIUC, the only thing little different here is that throttling is implemented by flusher thread. But it is still per device per cgroup. I think that is just a implementation detail whether we implement it in block layer, or in writeback or somewhere else. We can very well implement it in block layer and provide per bdi/per_group congestion flag in bdi so that flusher will stop pushing more IO if group on a bdi is congested (because IO is throttled). I think first important thing is to figure out what is minimal set of requirement (As jan said in another mail), which will solve wide variety of cases. I am trying to list some of points. - Throttling for buffered writes - Do we want per device throttling limits or global throttling limtis. - Exising direct write limtis are per device and implemented in block layer. - I personally think that both kind of limits might make sense. But a global limit for async write might make more sense at least for the workloads like backup which can run on a throttled speed. - Absolute throttling IO will make most sense on top level device in the IO stack. - For per device rate throttling, do we want a common limit for direct write and buffered write or a separate limit just for buffered writes. - Proportional IO for async writes - Will probably make most sense on bottom most devices in the IO stack (If we are able to somehow retain the submitter's context). - Logically it will make sense to keep sync and async writes in same group and try to provide fair share of disk between groups. Technically CFQ can do that but in practice I think it will be problematic. Writes of one group will take precedence of reads of another group. Currently any read is prioritized over buffered writes. So by splitting buffered writes in their own cgroups, they can serverly impact the latency of reads in another group. Not sure how many people really want to do that in practice. - Do we really need proportional IO for async writes. CFQ had tried implementing ioprio for async writes but it does not work. Should we just care about groups of sync IO and let all the async IO on device go in a single queue and lets make suere it is not starved while sync IO is going on. - I thought that most of the people cared about not impacting sync latencies badly while buffered writes are happening. Not many complained that buffered writes of one application should happen faster than other application. - If we agree that not many people require service differentation between buffered writes, then we probably don't have to do anything in this space and we can keep things simple. I personally prefer this option. Trying to provide proportional IO for async writes will make things complicated and we might not achieve much. - CFQ already does a very good job of prioritizing sync over async (at the cost of reduced throuhgput on fast devices). So what's the use case of proportion IO for async writes. Once we figure out what are the requirements, we can discuss the implementation details. Thanks Vivek -- 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
