Re: [PATCH 09/24] xfs: don't allow log IO to be throttled

Christoph Hellwig
2019-08-02T08:12:26Z

On Fri, Aug 02, 2019 at 09:58:49AM +1000, Dave Chinner wrote:
> Which simply reinforces the fact that that request type based
> throttling is a fundamentally broken architecture.
>
> > It feels awkward to have one set of prio inversion workarounds for io.*
> > and another for wbt. Jens, should we make an explicit one that doesn't
> > rely on magic side effects, or just decide that metadata is meta enough
> > to break all the rules?
>
> The problem isn't REQ_META blows throw the throttling, the problem
> is that different REQ_META IOs have different priority.
>
> IOWs, the problem here is that we are trying to infer priority from
> the request type rather than an actual priority assigned by the
> submitter. There is no way direct IO has higher priority in a
> filesystem than log IO tagged with REQ_META as direct IO can require
> log IO to make progress. Priority is a policy determined by the
> submitter, not the mechanism doing the throttling.
>
> Can we please move this all over to priorites based on
> bio->b_ioprio? And then document how the range of priorities are
> managed, such as:

Yes, we need to fix the magic deducted throttling behavior, especiall
the odd REQ_IDLE that in its various incarnations has been a massive
source of toruble and confusion. Not sure tons of priorities are
really helping, given that even hardware with priority level support
usually just supports about two priorit levels.