Re: [Xen-devel] [PATCH v2 03/23] x86: zero BSS using stosl instead of stosb

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/07/15 11:04, Jan Beulich wrote:
>>>> On 22.07.15 at 10:42, <andrew.cooper3@xxxxxxxxxx> wrote:
>> In the case of having aligned source and destination on a 16-byte
>> boundary (which we can trivially arrange), then ERMSB (to give it its
>> Intel name) and rep stosl differ only in the setup cost; they still
>> scale at the same rate for changes in length.
>>
>> Therefore, assuming we arrange for 16-byte alignment, using rep stosl
>> would appear to be a single 60ish cycle hit over using ERMSB, but would
>> be substantially more efficient than using rep stosb on a non-ERMSB system.
>>
>> Overall, I think 16 byte alignment and rep stosl is the best compromise.
> Or leaving such code alone, with the assumption that over time the
> setup cost (on a growing number of systems) outweighs the benefits
> (on a shrinking set).

The BSS is large - 295k on the last compile I have from staging.  The
setup cost is lost in the nose compared to the elapsed time to write
that many zeroes to memory.

Therefore, on an ERMBS-capable system, the two options will complete in
the same amount of time.

However, on all AMD hardware and Intel hardware older than IvyBridge,
rep stosl is 4 times faster than rep stosb.

~Andrew

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