[dm-devel] Re: dm-ioband + bio-cgroup benchmarks

[Andrea Righi <righi andrea gmail com>]

Vivek Goyal wrote:
> On Fri, Sep 19, 2008 at 08:20:31PM +0900, Hirokazu Takahashi wrote:
>>> To avoid creation of stacking another device (dm-ioband) on top of every
>>> device we want to subject to rules, I was thinking of maintaining an
>>> rb-tree per request queue. Requests will first go into this rb-tree upon
>>> __make_request() and then will filter down to elevator associated with the
>>> queue (if there is one). This will provide us the control of releasing
>>> bio's to elevaor based on policies (proportional weight, max bandwidth
>>> etc) and no need of stacking additional block device.
>> I think it's a bit late to control I/O requests there, since process
>> may be blocked in get_request_wait when the I/O load is high.
>> Please imagine the situation that cgroups with low bandwidths are
>> consuming most of "struct request"s while another cgroup with a high
>> bandwidth is blocked and can't get enough "struct request"s.
>>
>> It means cgroups that issues lot of I/O request can win the game.
>>
> 
> Ok, this is a good point. Because number of struct requests are limited
> and they seem to be allocated on first come first serve basis, so if a
> cgroup is generating lot of IO, then it might win.
> 
> But dm-ioband will face the same issue. Essentially it is also a request
> queue and it will have limited number of request descriptors. Have you 
> modified the logic somewhere for allocation of request descriptors to the
> waiting processes based on their weights? If yes, the logic probably can
> be implemented here too.

Maybe throttling dirty page ratio in memory could help to avoid this problem.
I mean, if a cgroup is exceeding the i/o limits do ehm... something.. also at
the balance_dirty_pages() level.

-Andrea