Re: [PATCH V3 02/16] block, bfq: add full hierarchical scheduling and cgroups support

[Paolo Valente]

> Il giorno 11 apr 2017, alle ore 23:47, Tejun Heo <tj@xxxxxxxxxx> ha scritto:
> 
> Hello,
> 
> On Tue, Apr 11, 2017 at 03:43:01PM +0200, Paolo Valente wrote:
>> From: Arianna Avanzini <avanzini.arianna@xxxxxxxxx>
>> 
>> Add complete support for full hierarchical scheduling, with a cgroups
>> interface. Full hierarchical scheduling is implemented through the
>> 'entity' abstraction: both bfq_queues, i.e., the internal BFQ queues
>> associated with processes, and groups are represented in general by
>> entities. Given the bfq_queues associated with the processes belonging
>> to a given group, the entities representing these queues are sons of
>> the entity representing the group. At higher levels, if a group, say
>> G, contains other groups, then the entity representing G is the parent
>> entity of the entities representing the groups in G.
>> 
>> Hierarchical scheduling is performed as follows: if the timestamps of
>> a leaf entity (i.e., of a bfq_queue) change, and such a change lets
>> the entity become the next-to-serve entity for its parent entity, then
>> the timestamps of the parent entity are recomputed as a function of
>> the budget of its new next-to-serve leaf entity. If the parent entity
>> belongs, in its turn, to a group, and its new timestamps let it become
>> the next-to-serve for its parent entity, then the timestamps of the
>> latter parent entity are recomputed as well, and so on. When a new
>> bfq_queue must be set in service, the reverse path is followed: the
>> next-to-serve highest-level entity is chosen, then its next-to-serve
>> child entity, and so on, until the next-to-serve leaf entity is
>> reached, and the bfq_queue that this entity represents is set in
>> service.
>> 
>> Writeback is accounted for on a per-group basis, i.e., for each group,
>> the async I/O requests of the processes of the group are enqueued in a
>> distinct bfq_queue, and the entity associated with this queue is a
>> child of the entity associated with the group.
>> 
>> Weights can be assigned explicitly to groups and processes through the
>> cgroups interface, differently from what happens, for single
>> processes, if the cgroups interface is not used (as explained in the
>> description of the previous patch). In particular, since each node has
>> a full scheduler, each group can be assigned its own weight.
> 
> Can we please hold off on cgroup support for now?  I've been trying to
> chase down cpu scheduler latency issues lately and have some doubts
> about implementing cgroup support by simply nesting the timelines like
> this.
> 

Hi Tejun,
could you elaborate a bit more on this?  I mean, cgroups support has
been in BFQ (and CFQ) for almost ten years, perfectly working as far
as I know.  Of course it is perfectly working in terms of I/O and not
of CPU bandwidth distribution; and, for the moment, it is effective
only for devices below 30-50KIOPS.  What's the point in throwing
(momentarily?) away such a fundamental feature?  What am I missing?

Thanks,
Paolo


> Thanks
> 
> -- 
> tejun