Re: shadow prefetch

Dong, Eddie
2008-01-02T12:56:45Z

>Dong, Eddie wrote:
>> Current shadow code do prefetch in FNAME(prefetch_page), but
>it is only
>> used
>> to choose shadow_notrap_nonpresent_pte or shadow_trap_nonpresent_pte.
>>
>> At least for L1 shadow, prefetching to get exact shadow L1 pte won't
>> cause
>> performance regression (though handling time increases
>slightly ) since
>> it
>> won't generate a new write protected page.
>>
>>
>I'm not sure that "slightly" is accurate, we need to fetch 512 or 1024
>entries. On the other hand, we have the benefit of batching
>(a call to
>get_user_pages() with n = 512 is much faster than 512 calls. I think

Yes, that means we have to copy the guest page to kernel buffer.

>that this will improve fork()s, but reduce performance with
>sparsely-accessed memory maps, or with fork() followed by exec()
>immediately.

I saw pf_fixed reduced. Specifically the shadow fault is reduced, but
overal perf gain with kernel build is minor.

>
>Only benchmarking can tell if it is an overall win.
>
>Xen limits the prefetch to a subset of the pages, maybe that's
>a better
>approach.
>
>
>> for those L2+ shadow page tables, we can do similar for those pointed
>> gfn which
>> is already shadowed
>
>I think L2s are very rare compared to L1 page tables, so any benefit
>would be minor.
>
Agree it happens rare.
Eddie