Re: Populating multiple ptes at fault time

[Jeremy Fitzhardinge <jeremy@goop.org>]

Martin Bligh wrote:
>>>> My patches were only for anonymous pages not for file backed because readahead
>>>> is available for file backed mappings.
>>>>         
>>> Do we populate the PTEs though? I didn't think that was batched, but I
>>> might well be wrong.
>>>       
>> We do not populate the PTEs and AFAICT PTE population was assumed not to be
>> performance critical since the backing media is comparatively slow.
>>     
>
> I think the times when this matters are things like glibc, which are
> heavily shared -
> we were only 'prefaulting' when the pagecache was already there. So it's a case
> for a "readahead like algorithm", not necessarily a direct hook.
>   

Yes.  My thought was that there should be very little cost to
opportunistically populating the pte for a page which is already
resident anyway.

> Anonymous pages seem much riskier, as presumably there's a no backing page
> except in the fork case.
>
> I presume the reason Jeremy is interested is because his pagefaults are more
> expensive than most (under virtualization), so he may well find a
> different tradeoff
> than I did (try running kernbench?)
>   

Right.  The faults themselves are more or less the same as the native
case, but setting a pte requires a hypercall compared to a memory write
in the native case.  But I can set any number of ptes in one hypercall,
so batching would amortize the cost.

    J