Re: [RFC] [PATCH 0/6] Provide cgroup isolation for buffered writes.

Justin TerAvest
2011-03-11T16:07:43Z

On Thu, Mar 10, 2011 at 6:47 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> On Wed, Mar 09, 2011 at 10:04:11AM -0800, Justin TerAvest wrote:
>> On Tue, Mar 8, 2011 at 2:50 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
>> > On Tue, Mar 08, 2011 at 05:43:25PM -0500, Vivek Goyal wrote:
>> >> On Tue, Mar 08, 2011 at 01:20:50PM -0800, Justin TerAvest wrote:
>> >> > This patchset adds tracking to the page_cgroup structure for which cgroup has
>> >> > dirtied a page, and uses that information to provide isolation between
>> >> > cgroups performing writeback.
>> >> >
>> >>
>> >> Justin,
>> >>
>> >> So if somebody is trying to isolate a workload which does bunch of READS
>> >> and lots of buffered WRITES, this patchset should help in the sense that
>> >> all the heavy WRITES can be put into a separate cgroup of low weight?
>> >>
>> >> Other application which are primarily doing READS, direct WRITES or little
>> >> bit of buffered WRITES should still get good latencies if heavy writer
>> >> is isolated in a separate group?
>> >>
>> >> If yes, then this piece standalone can make sense. And once the other
>> >> piece/patches of memory cgroup dirty ratio and cgroup aware buffered
>> >> writeout come in, then one will be able to differentiate buffered writes
>> >> of different groups.
>> >
>> > Thinking more about it, currently anyway SYNC preempts the ASYNC. So the
>> > question would be will it help me enable get better isolation latencies
>> > of READS agains buffered WRITES?
>>
>> Ah! Sorry, I left out a patch that disables cross-group preemption.
>> I'll add that to the patchset and email out v2 soon.
>
> Well, what I was referring to that even in current code sync preempts
> all async in CFQ. So it looks like this patchset will not help get
> better latencies in presence of WRITES?

Hi Vivek,

I should have been more clear. I forgot to include a patch that
changes the behavior of that preemption. I haven't mailed out v2 yet
because I was also writing a change to put the css_id in pc->flags
instead of its own field.

The preemption change would look like:

Previously, a sync queue in can preempt an async queue in another
cgroup. This changes that behavior to disallow such preemption.

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index ab7a216..0494c0c 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3390,6 +3390,9 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue
if (!cfqq)
return false;

+ if (new_cfqq->cfqg != cfqq->cfqg)
+ return false;
+
if (cfq_class_idle(new_cfqq))
return false;

@@ -3409,9 +3412,6 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue
if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
return true;

- if (new_cfqq->cfqg != cfqq->cfqg)
- return false;

I will include the test results that show that isolation is also
improved between a reader and a buffered writer.

Thanks,
Justin

>
> The only place it can help is that one is looking for service differentation
> between two or more buffered write streams. For that we need to fix
> upper layers first.
>
> Vivek
>