Re: [PATCH v5 0/4] Add support Weighted Round Robin for blkcg and nvme

Tejun Heo [off-list ref] 于2020年3月31日周二 下午10:36写道：

Hello, Weiping.

On Tue, Mar 31, 2020 at 02:17:06PM +0800, Weiping Zhang wrote:

quoted

Recently I do some cgroup io weight testing,
https://github.com/dublio/iotrack/wiki/cgroup-io-weight-test
I think a proper io weight policy
should consider high weight cgroup's iops, latency and also take whole
disk's throughput
into account, that is to say, the policy should do more carfully trade
off between cgroup's
IO performance and whole disk's throughput. I know one policy cannot
do all things perfectly,
but from the test result nvme-wrr can work well.

That's w/o iocost QoS targets configured, right? iocost should be able to
achieve similar results as wrr with QoS configured.

Yes, I have not set Qos target.

quoted

From the following test result, nvme-wrr work well for both cgroup's
latency, iops, and whole
disk's throughput.

As I wrote before, the issues I see with wrr are the followings.

* Hardware dependent. Some will work ok or even fantastic. Many others will do
  horribly.

* Lack of configuration granularity. We can't configure it granular enough to
  serve hierarchical configuration.

* Likely not a huge problem with the deep QD of nvmes but lack of queue depth
  control can lead to loss of latency control and thus loss of protection for
  low concurrency workloads when pitched against workloads which can saturate
  QD.

All that said, given the feature is available, I don't see any reason to not
allow to use it, but I don't think it fits the cgroup interface model given the
hardware dependency and coarse granularity. For these cases, I think the right
thing to do is using cgroups to provide tagging information - ie. build a
dedicated interface which takes cgroup fd or ino as the tag and associate
configurations that way. There already are other use cases which use cgroup this
way (e.g. perf).

Do you means drop the "io.wrr" or "blkio.wrr" in cgroup, and use a
dedicated interface
like /dev/xxx or /proc/xxx?

I see the perf code:
struct fd f = fdget(fd)
struct cgroup_subsys_state *css =
css_tryget_online_from_dir(f.file->f_path.dentry,
        &perf_event_cgrp_subsys);

Looks can be applied to block cgroup in same way.

Thanks your help.