Re: [PATCH V2 3/3] blockcg: export latency info for each cgroup

[Shaohua Li <shli@kernel.org>]

On Wed, Oct 11, 2017 at 01:35:51AM +0800, weiping zhang wrote:
> On Fri, Oct 06, 2017 at 05:56:01PM -0700, Shaohua Li wrote:
> > From: Shaohua Li <shli@fb.com>
> > 
> > Export the latency info to user. The latency is a good sign to indicate
> > if IO is congested or not. User can use the info to make decisions like
> > adjust cgroup settings.
> Hi Shaohua,
> How to check IO congested or not by latency ? Different SSD has
> different latency especially when mixed sequence and random IO
> operatons.

There is no magic here, you should know the SSD characteristics first. The idea
is observing the latency when the system isn't overcommited, for example,
running the workload in a one-cgroup setup, then using the observed latency to
guide configuration for multi-cgroup settings or check if IO is healthy in
cgroups.
 
> > 
> > Existing io.stat shows accumulated IO bytes and requests, but
> > accumulated value for latency doesn't make much sense. This patch
> How to understand "accumulated value for latency doesn't make much
> sense" could you give an example? 

rbytes and wbytes in io.stat are accumulated value. If such value of latency
isn't valuable.

> I think iostat's r_await and w_await
> are nearly equal to rlat_mean and wlat_mean if there is no too much
> request starved.

yes, pretty similar. io.stat is per-cgroup though

Thanks,
Shaohua