Hi, cgroup still lacks a good iocontroller. CFQ works well for hard disk, but not much for SSD. This patch set try to add a conservative limit for blk-throttle. It isn't a proportional scheduling, but can help prioritize cgroups. There are several advantages we choose blk-throttle: - blk-throttle resides early in the block stack. It works for both bio and request based queues. - blk-throttle is light weight in general. It still takes queue lock, but it's not hard to implement a per-cpu cache and remove the lock contention. - blk-throttle doesn't use 'idle disk' mechanism, which is used by CFQ/BFQ. The mechanism is proved to harm performance for fast SSD.
The patch set add a new io.low limit for blk-throttle. It's only for cgroup2. The existing io.max is a hard limit throttling. cgroup with a max limit never dispatch more IO than its max limit. While io.low is a best effort throttling. cgroups with 'low' limit can run above their 'low' limit at appropriate time. Specifically, if all cgroups reach their 'low' limit, all cgroups can run above their 'low' limit. If any cgroup runs under its 'low' limit, all other cgroups will run according to their 'low' limit. So the 'low' limit could act as two roles, it allows cgroups using free bandwidth and it protects cgroups from their 'low' limit. An example usage is we have a high prio cgroup with high 'low' limit and a low prio cgroup with low 'low' limit. If the high prio cgroup isn't running, the low prio can run above its 'low' limit, so we don't waste the bandwidth. When the high prio cgroup runs and is below its 'low' limit, low prio cgroup will run under its 'low' limit. This will protect high prio cgroup to get more resources. The implementation is simple. The disk queue has a state machine. We have 2 states LIMIT_LOW and LIMIT_MAX. In each disk state, we throttle cgroups according to the limit of the state. That is io.low limit for LIMIT_LOW state, io.max limit for LIMIT_MAX. The disk state can be upgraded/downgraded between LIMIT_LOW and LIMIT_MAX according to the rule aboe. Initially disk state is LIMIT_MAX. And if no cgroup sets io.low, the disk state will remain in LIMIT_MAX state. Systems with only io.max set will find nothing changed with the patches. The first 10 patches implement the basic framework. Add interface, handle upgrade and downgrade logic. The patch 10 detects a special case a cgroup is completely idle. In this case, we ignore the cgroup's limit. The patch 11-18 adds more heuristics. The basic framework has 2 major issues. 1. fluctuation. When the state is upgraded from LIMIT_LOW to LIMIT_MAX, the cgroup's bandwidth can change dramatically, sometimes in a way we are not expected. For example, one cgroup's bandwidth will drop below its io.low limit very soon after a upgrade. patch 10 has more details about the issue. 2. idle cgroup. cgroup with a io.low limit doesn't always dispatch enough IO. In above upgrade rule, the disk will remain in LIMIT_LOW state and all other cgroups can't dispatch more IO above their 'low' limit. Hence there is waste. patch 11 has more details about the issue. For issue 1, we make cgroup bandwidth increase/decrease smoothly after a upgrade/downgrade. This will reduce the chance a cgroup's bandwidth drop under its 'low' limit rapidly. The smoothness means we could waste some bandwidth in the transition though. But we must pay something for sharing. The issue 2 is very hard. We introduce two mechanisms for this. One is 'idle time' or 'think time' borrowed from CFQ. If a cgroup's average idle time is high, we treat it's idle and its 'low' limit isn't respected. Please see patch 12 - 14 for details. The other is 'latency target'. If a cgroup's io latency is low, we treat it's idle and its 'low' limit isn't resptected. Please see patch 15 - 18 for fetails. Both mechanisms only happen when a cgroup runs below its 'low' limit. The disadvantages of blk-throttle is it exports a kind of low level knobs. Configuration would not be easy for normal users. It would be powerful for experienced users though. More tuning is required of course, but otherwise this works well. Please review, test and consider merge. Thanks, Shaohua V5->V6: - Change default setting for io.low limit. It's 0 now, which makes more sense - The default setting for latency is still 0, the default setting for idle time becomes bigger. So with the default settings, cgroups have small latency but disk sharing could be harmed - Addressed other issues pointed out by Tejun V4->V5, basically address Tejun's comments: - Change interface from 'io.high' to 'io.low' so consistent with memcg - Change interface for 'idle time' and 'latency target' - Make 'idle time' per-cgroup-disk instead of per-cgroup - Chnage interface name for 'throttle slice'. It's not a real slice - Make downgrade smooth too - Make latency sampling work for both bio and request based queue - Change latency estimation method from 'line fitting' to 'bucket based calculation' - Rebase and fix other problems Issue pointed out by Tejun isn't fixed yet: - .pd_offline_fn vs .pd_free_fn. .pd_free_fn seems too late to change states http://marc.info/?l=linux-kernel&m=148183437022975&w=2 V3->V4: - Add latency target for cgroup - Fix bugs http://marc.info/?l=linux-block&m=147916216512915&w=2 V2->V3: - Rebase - Fix several bugs - Make harddisk think time threshold bigger http://marc.info/?l=linux-kernel&m=147552964708965&w=2 V1->V2: - Drop io.low interface for simplicity and the interface isn't a must-have to prioritize cgroups. - Remove the 'trial' logic, which creates too much fluctuation - Add a new idle cgroup detection - Other bug fixes and improvements http://marc.info/?l=linux-block&m=147395674732335&w=2 V1: http://marc.info/?l=linux-block&m=146292596425689&w=2 Shaohua Li (18): blk-throttle: use U64_MAX/UINT_MAX to replace -1 blk-throttle: prepare support multiple limits blk-throttle: add .low interface blk-throttle: configure bps/iops limit for cgroup in low limit blk-throttle: add upgrade logic for LIMIT_LOW state blk-throttle: add downgrade logic blk-throttle: make sure expire time isn't too big blk-throttle: make throtl_slice tunable blk-throttle: choose a small throtl_slice for SSD blk-throttle: detect completed idle cgroup blk-throttle: make bandwidth change smooth blk-throttle: add a simple idle detection blk-throttle: add interface to configure idle time threshold blk-throttle: ignore idle cgroup limit blk-throttle: add interface for per-cgroup target latency block: track request size in blk_issue_stat blk-throttle: add a mechanism to estimate IO latency blk-throttle: add latency target support Documentation/block/queue-sysfs.txt | 6 + block/bio.c | 2 + block/blk-core.c | 2 +- block/blk-mq.c | 2 +- block/blk-stat.c | 11 +- block/blk-stat.h | 29 +- block/blk-sysfs.c | 12 + block/blk-throttle.c | 961 +++++++++++++++++++++++++++++++++--- block/blk-wbt.h | 10 +- block/blk.h | 9 + include/linux/blk_types.h | 10 +- 11 files changed, 959 insertions(+), 95 deletions(-) -- 2.9.3 -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
