Re: [PATCH 1 of 2 RFC] xen, pod: Zero-check recently populated pages (checklast)

[George Dunlap <george.dunlap@eu.citrix.com>]

On 14/06/12 10:07, Tim Deegan wrote:
>
>
> At 13:02 +0100 on 08 Jun (1339160536), Jan Beulich wrote:
>>>>> On 08.06.12 at 13:45, George Dunlap<george.dunlap@eu.citrix.com>  wrote:
>>> --- a/xen/include/asm-x86/p2m.h
>>> +++ b/xen/include/asm-x86/p2m.h
>>> @@ -287,6 +287,9 @@ struct p2m_domain {
>>>           unsigned         reclaim_super; /* Last gpfn of a scan */
>>>           unsigned         reclaim_single; /* Last gpfn of a scan */
>>>           unsigned         max_guest;    /* gpfn of max guest demand-populate */
>>> +#define POD_HISTORY_MAX 128
>>> +        unsigned         last_populated[POD_HISTORY_MAX]; /* gpfn of last guest page demand-populated */
> This is the gpfns of the last 128 order-9 superpages populated, right?
Ah, yes -- just order 9.
> Also, this line is>80 columns - I think I saw a few others in this series.
I'll go through and check, thanks.
>
>> unsigned long?
>>
>> Also, wouldn't it be better to allocate this table dynamically, at
>> once allowing its size to scale with the number of vCPU-s in the
>> guest?
> You could even make it a small per-vcpu array, assuming that the parallel
> scrubbing will be symmetric across vcpus.
I can't remember exactly what I found here (this was last summer I was 
doing the tests); it may be that Windows creates a bunch of tasks which 
may migrate to various cpus.  If that were the case, a global list would 
be better than per-vcpu lists.

The problem with dynamically scaling the list is that I don't have a 
heuristic to hand for how to scale it.

In both cases, it's not unlikely that making a change without testing 
will significantly reduce the effectiveness of the patch.  Would you 
rather hold off and wait until I can get a chance to run my benchmarks 
again (which may miss the 4.2 cycle), or accept a tidied-up version of 
this patch first, and hope to get a revised method (using dynamic 
scaling or per-vcpu arrays) in before 4.2, but for sure by 4.3?

  -George