Re: Populating multiple ptes at fault time

[Jeremy Fitzhardinge <jeremy@goop.org>]

Chris Snook wrote:
> Is it still expensive when you're using nested page tables?

No, nested pagetables are the same as native to update, so the main
benefit in that case is the reduction of faults.

> We already have rather well-tested code in the VM to detect fault
> patterns, complete with userspace hints to set readahead policy.  It
> seems to me that if we're going to read nearby pages into pagecache,
> we might as well actually map them at the same time.  Duplicating the
> readahead code is probably a bad idea.

Right, that was my point.  I'm assuming that that machinery already
exists and would be available for use in this case.

>> Minor faults are easier; if the page already exists in memory, we should
>> just create mappings to it.  If neighbouring pages are also already
>> present, then we can can cheaply create mappings for them too.
>
> If we're mapping pagecache, then sure, this is really cheap, but
> speculatively allocating anonymous pages will hurt, badly, on many
> workloads.

OK, makes sense.  Does the access pattern detecting code measure access
patterns to anonymous mappings?

>> This seems like an obvious idea, so I'm wondering if someone has
>> prototyped it already to see what effects there are.  In the native
>> case, pte updates are much cheaper, so perhaps it doesn't help much
>> there, though it would potentially reduce the number of faults
>> needed. But I think there's scope for measurable benefits in the
>> virtual case.
>
> Sounds like something we might want to enable conditionally on the use
> of pv_ops features.

Perhaps, but I'd rather avoid it.  I'm hoping this is something we could
do that has - at worst - no effect on the native case, while improving
the virtual case.  The test matrix is already large enough without
adding another stateful switch.  After all, any side effect which makes
it a bad idea for the native case will probably be bad enough to
overwhelm any benefit in the virtual case.

    J