Re: [PATCH] SUNRPC: restore fair scheduling to priority queues.

[Trond Myklebust]

On Tue, Dec 15, 2015 at 10:10 PM, NeilBrown <neilb@xxxxxxxx> wrote:
> On Wed, Dec 16 2015, Trond Myklebust wrote:
>
>> On Tue, Dec 15, 2015 at 6:44 PM, NeilBrown <neilb@xxxxxxxx> wrote:
>>>
>>> Commit: c05eecf63610 ("SUNRPC: Don't allow low priority tasks to pre-empt higher priority ones")
>>>
>>> removed the 'fair scheduling' feature from SUNRPC priority queues.
>>> This feature caused problems for some queues (send queue and session slot queue)
>>> but is still needed for others, particularly the tcp slot queue.
>>>
>>> Without fairness, reads (priority 1) can starve background writes
>>> (priority 0) so a streaming read can cause writeback to block
>>> indefinitely.  This is not easy to measure with default settings as
>>> the current slot table size is much larger than the read-ahead size.
>>> However if the slot-table size is reduced (seen when backporting to
>>> older kernels with a limited size) the problem is easily demonstrated.
>>>
>>> This patch conditionally restores fair scheduling.  It is now the
>>> default unless rpc_sleep_on_priority() is called directly.  Then the
>>> queue switches to strict priority observance.
>>>
>>> As that function is called for both the send queue and the session
>>> slot queue and not for any others, this has exactly the desired
>>> effect.
>>>
>>> The "count" field that was removed by the previous patch is restored.
>>> A value for '255' means "strict priority queuing, no fair queuing".
>>> Any other value is a could of owners to be processed before switching
>>> to a different priority level, just like before.
<snip>
>> Are we sure there is value in keeping FLUSH_LOWPRI for background writes?
>
> There is currently also FLUSH_HIGHPRI for "for_reclaim" writes.
> Should they be allowed to starve reads?
>
> If you treated all reads and writed the same, then I can't see value in
> restoring fair scheduling.   If there is any difference, then I suspect
> we do need the fairness.

I disagree. Reclaiming memory should always be able to pre-empt
"interactive" features such as read. Everything goes down the toilet
when we force the kernel into situations where it needs to swap.

Cheers
  Trond
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