Re: [RFC PATCH 0/3] cgroup: fsio throttle controller

[Paolo Valente]

> Il giorno 18 gen 2019, alle ore 12:10, Andrea Righi <righi.andrea@xxxxxxxxx> ha scritto:
> 
> On Fri, Jan 18, 2019 at 12:04:17PM +0100, Paolo Valente wrote:
>> 
>> 
>>> Il giorno 18 gen 2019, alle ore 11:31, Andrea Righi <righi.andrea@xxxxxxxxx> ha scritto:
>>> 
>>> This is a redesign of my old cgroup-io-throttle controller:
>>> https://lwn.net/Articles/330531/
>>> 
>>> I'm resuming this old patch to point out a problem that I think is still
>>> not solved completely.
>>> 
>>> = Problem =
>>> 
>>> The io.max controller works really well at limiting synchronous I/O
>>> (READs), but a lot of I/O requests are initiated outside the context of
>>> the process that is ultimately responsible for its creation (e.g.,
>>> WRITEs).
>>> 
>>> Throttling at the block layer in some cases is too late and we may end
>>> up slowing down processes that are not responsible for the I/O that
>>> is being processed at that level.
>>> 
>>> = Proposed solution =
>>> 
>>> The main idea of this controller is to split I/O measurement and I/O
>>> throttling: I/O is measured at the block layer for READS, at page cache
>>> (dirty pages) for WRITEs, and processes are limited while they're
>>> generating I/O at the VFS level, based on the measured I/O.
>>> 
>> 
>> Hi Andrea,
>> what the about the case where two processes are dirtying the same
>> pages?  Which will be charged?
>> 
>> Thanks,
>> Paolo
> 
> Hi Paolo,
> 
> in this case only the first one will be charged for the I/O activity
> (the one that changes a page from clean to dirty). This is probably not
> totally fair in some cases, but I think it's a good compromise,

Absolutely, I just wanted to better understand this point.

> at the
> end rewriting the same page over and over while it's already dirty
> doesn't actually generate I/O activity, until the page is flushed back
> to disk.
> 

Right.

Thanks,
Paolo

> Obviously I'm open to other better ideas and suggestions.
> 
> Thanks!
> -Andrea