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

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On Fri, Aug 02, 2019 at 02:11:53PM +0000, Chris Mason wrote:
> Yes and no.  At some point important FS threads have the potential to 
> wait on every single REQ_META IO on the box, so every single REQ_META IO 
> has the potential to create priority inversions.

[...]

> Tejun reminded me that in a lot of ways, swap is user IO and it's 
> actually fine to have it prioritized at the same level as user IO.  We 
> don't want to let a low prio app thrash the drive swapping things in and 
> out all the time, and it's actually fine to make them wait as long as 
> other higher priority processes aren't waiting for the memory.  This 
> depends on the cgroup config, so wrt your current patches it probably 
> sounds crazy, but we have a lot of data around this from the fleet.

swap is only user IO if we're doing the swapping in response to an
allocation done on behalf of a user thread.  If one of the above-mentioned
important FS threads does a memory allocation which causes swapping,
that priority needs to be inherited by the IO.



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