Re: [Xen-devel] Converting heap page_infos to contiguous virtual

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/07/16 15:53, Boris Ostrovsky wrote:
> On 07/14/2016 09:29 AM, Andrew Cooper wrote:
>> However, I would recommend getting something functioning first, before
>> trying to optimise it.
> There are two fairly independent parts to improving scrubbing: one is
> making it asynchronous and second is improving clear_page() performance.
> Whole-RAM mapping is needed for the latter.
>
>> There is probably a lot to be gained simply by improving clear_page().
> The biggest improvement comes from switching to AVX(2) when available.
> It's been a while since I ran those tests so I will have to re-measure
> it but my recollection is that 4K was too small to see significant changes.

There is also the new `clzero` on AMD Zen processors, which looks like
an interesting option which doesn't involve context switching SIMD state.

>
> A potential improvement might come from dropping (or, rather, deferring)
> sfence in clear_page_sse2. I don't know how much this would buy us though.

The sfence is mandatory because of the movnti.  It cannot be dropped, or
the zeroes might be stuck in the Write Combining buffer when something
else comes to access the page.

~Andrew

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