On Tue, 2006-02-28 at 20:33 +0100, Andi Kleen wrote:
> Anyways if MFENCE improved performance you're probably relying
> on some very specific artifact of the microarchitecture of your
> CPU or Northbridge. I don't think it's a architecurally guaranteed
> feature.
I looked this up, and you appear to be wrong here.
Here's the appropriate quote from page 246 of the PDF of "AMD64
Architecture Programmer's Manual Volume 2: System Programming":
http://www.amd.com/us-en/assets/content_type/DownloadableAssets/dwamd_24593.pdf
Section 7.4.1 specifically describes what happens to write buffers:
[...] the processor completely empties the write buffer by
writing the contents to memory as a result of performing any of
the following operations:
SFENCE Instruction
Executing a store-fence (SFENCE) instruction forces all memory
writes before the SFENCE (in program order) to be written into
memory before memory writes that follow the SFENCE instruction.
The memory-fence (MFENCE) instruction has a similar effect, but
it forces the ordering of loads in addition to stores.
[...]
So in fact SFENCE is the appropriate, architecturally guaranteed, thing
for us to be doing on x86_64.
With respect to Ben's contention that wmb() will suffice instead, that
isn't true, either, even on x86-class hardware. The writes absolutely
travel over the HT bus in non-ascending order on AMD64 systems unless we
fence them, and we've verified this using a HT bus analyser.
<b
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