svn commit: r303583 - head/sys/amd64/amd64

[Bruce Evans]

On Sun, 31 Jul 2016, Slawa Olhovchenkov wrote:

> On Sun, Jul 31, 2016 at 11:11:25PM +1000, Bruce Evans wrote:
>
>> Misalignment of this loop made it almost twice as slow on old Turion2 with
>> slow DDR2 memory.  It made no difference on Haswell.  I added an extra
>> movnti, but that makes little or no differences.  2 more movnti's wouldn't
>> fit in a 16-byte cache line so are slower unless even more care is taken
>> with alignment (or with less care, 4 with misalignment are not less than
>> twice as slow as 1 with alignment).
>>
>> I thought that alignment and unrolling didn't matter here, because movnti
>> has to wait for memory and almost any loop runs fast enough to keep up.
>> The timing on my old system is something like: CPUs at 2 GHz; main memory
>> at 4 GB/sec; movnti is only 4 bytes wide on i386 (so this problem
>> only affects i386, at least with slow memory).  So sustaining 4 GB/sec
>> requires 1 G movnti's/sec, so the loop needs to run at 2 cycles/iteration
>> to keep up.  But when it is misaligned, it runs at 3-4 cycles/iteration.
>> Alignment makes it take about 2, and the extra movnti is for safety and
>> to work with faster memory.
>>
>> On Haswell with CPUs at 4 GHz, 2 cycles/iteration gives 8 GB/sec on
>> i386 and 16 GB/sec on amd64 with wider movnti.  IIRC, 16 GB/sec is about
>> the main memory speed so nothing better is possible but just 1 extra
>> movnti gives more with faster memory.  This is just worse than bzero()
>
> What about modern system with 120 GB/sec main memory speed?

Is there such a system?  It would have main memory almost twice as fast
as Haswell L2 and almost half as fast as Haswell L1.

My fastest memory actually does 20001 MB/s according to old memtest
and that is about right according to other tests.

Bruce