Re: [PATCH RFC 10/22] block, bfq: add full hierarchical scheduling and cgroups support

[Paolo]

Il 25/04/2016 21:24, Tejun Heo ha scritto:
> Hello, Paolo.

Hi

> On Sat, Apr 23, 2016 at 09:07:47AM +0200, Paolo Valente wrote:
> > There is certainly something I donât know here, because I donât
> > understand why there is also a workqueue containing root-group I/O
> > all the time, if the only process doing I/O belongs to a different
> > (sub)group.
>
> Hmmm... maybe metadata updates?

That's what I thought in the first place. But one half or one third of
the IOs sounded too much for metadata (the percentage varies over time
during the test). And root-group IOs are apparently large. Here is an
excerpt from the output of

grep -B 1 insert_request trace

    kworker/u8:4-116   [002] d...   124.349971:   8,0    I   W 3903488 
+ 1024 [kworker/u8:4]
    kworker/u8:4-116   [002] d...   124.349978:   8,0    m   N cfq409A 
 / insert_request
--
    kworker/u8:4-116   [002] d...   124.350770:   8,0    I   W 3904512 
+ 1200 [kworker/u8:4]
    kworker/u8:4-116   [002] d...   124.350780:   8,0    m   N cfq96A 
/seq_write insert_request
--
    kworker/u8:4-116   [002] d...   124.363911:   8,0    I   W 3905712 
+ 1888 [kworker/u8:4]
    kworker/u8:4-116   [002] d...   124.363916:   8,0    m   N cfq409A 
 / insert_request
--
    kworker/u8:4-116   [002] d...   124.364467:   8,0    I   W 3907600 
+ 352 [kworker/u8:4]
    kworker/u8:4-116   [002] d...   124.364474:   8,0    m   N cfq96A 
/seq_write insert_request
--
    kworker/u8:4-116   [002] d...   124.369435:   8,0    I   W 3907952 
+ 1680 [kworker/u8:4]
    kworker/u8:4-116   [002] d...   124.369439:   8,0    m   N cfq96A 
/seq_write insert_request
--
    kworker/u8:4-116   [002] d...   124.369441:   8,0    I   W 3909632 
+ 560 [kworker/u8:4]
    kworker/u8:4-116   [002] d...   124.369442:   8,0    m   N cfq96A 
/seq_write insert_request
--
    kworker/u8:4-116   [002] d...   124.373299:   8,0    I   W 3910192 
+ 1760 [kworker/u8:4]
    kworker/u8:4-116   [002] d...   124.373301:   8,0    m   N cfq409A 
 / insert_request
--
    kworker/u8:4-116   [002] d...   124.373519:   8,0    I   W 3911952 
+ 480 [kworker/u8:4]
    kworker/u8:4-116   [002] d...   124.373522:   8,0    m   N cfq96A 
/seq_write insert_request
--
    kworker/u8:4-116   [002] d...   124.381936:   8,0    I   W 3912432 
+ 1728 [kworker/u8:4]
    kworker/u8:4-116   [002] d...   124.381937:   8,0    m   N cfq409A 
/ insert_request


> > Anyway, if this is expected, then there is no reason to bother you
> > further on it. In contrast, the actual problem I see is the
> > following. If one third or half of the bios belong to a different
> > group than the writer that one wants to isolate, then, whatever
> > weight is assigned to the writer group, we will never be able to let
> > the writer get the desired share of the time (or of the bandwidth
> > with bfq and all quasi-sequential workloads). For instance, in the
> > scenario that you told me to try, the writer will never get 50% of
> > the time, with any scheduler. Am I missing something also on this?
>
> While a worker may jump across different cgroups, the IOs are still
> coming from somewhere and if the only IO generator on the machine is
> the test dd, the bios from that cgroup should dominate the IOs.  I
> think it'd be helpful to investigate who's issuing the root cgroup
> IOs.

Ok (if there is some quick way to get this information without
instrumenting the code, then any suggestion or pointer is welcome).

Thanks,
Paolo

> Thanks.