Patch to use fence instructions

[Martin Nilsson]

John Baldwin wrote:
> On Wednesday 21 September 2005 07:45 am, Martin Nilsson wrote:
> 
>>John Baldwin wrote:
>>
>>>This patch changes the atomic operations and bus space barriers to use
>>>the x86 fence instructions.  Please test, thanks!
>>>
>>>http://www.FreeBSD.org/~jhb/patches/amd64_fences.patch
>>
>>What kind of performance improvements can we expect with this patch?
>>Is it worthwile to compare performance on dualcore Pentium D with
>>sysbench before and after this patch? Does it affect threads & mutex
>>performance?
>>
>>Sysbench is a benchmark specially made to determine lowlevel performance
>>important for MySQL and be found here: http://sysbench.sourceforge.net/
> 
> 
> I'm not sure what improvements it would provide (I don't have any amd64 
> hardware to test on anyway).  I believe that in some microbenchmarks bde@ 
> found that just using lfence or sfence was only about half the cost of using 
> the 'lock' prefix.  Thus, things like atomic_store_rel (used in mutexes) 
> might perform better.

I have tested the patch but I'm not able to see any difference with the 
mutex & threads tests in sysbench. On the other hand I'm not seeing any 
regressions either and everything seems to work OK. Can you suggest a 
better low-level test?

/Martin