On Fri, Jan 18, 2019 at 12:04:17PM +0100, Paolo Valente wrote:

quoted

Il giorno 18 gen 2019, alle ore 11:31, Andrea Righi [off-list ref] 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, 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.

Obviously I'm open to other better ideas and suggestions.

Thanks!
-Andrea