On Thu, Mar 03, 2016 at 04:27:39PM +0100, Ingo Molnar wrote: > > * Dexuan Cui <[email protected]> wrote: > > > Hi, > > My understanding about arch/x86/include/asm/barrier.h is: obviously Linux > > more likes {L,S,M}FENCE -- Locked ADD is only used in x86_32 platforms that > > don't support XMM2. > > > > However, it looks people say Locked Add is much faster than the FENCE > > instructions, even on modern Intel CPUs like Haswell, e.g., please see > > the three sources: > > > > " 11.5.1 Locked Instructions as Memory Barriers > > Optimization > > Use locked instructions to implement Store/Store and Store/Load barriers. > > " > > http://support.amd.com/TechDocs/47414_15h_sw_opt_guide.pdf > > > > "lock addl %(rsp), 0 is a better solution for StoreLoad barrier ": > > http://shipilev.net/blog/2014/on-the-fence-with-dependencies/ > > > > "...locked instruction are more efficient barriers...": > > http://www.pvk.ca/Blog/2014/10/19/performance-optimisation-~-writing-an-essay/ > > > > I also found that FreeBSD prefers Locked Add. > > > > So, I'm curious why Linux prefers MFENCE. > > I guess I may be missing something. > > > > I tried to google the question, but didn't find an answer. > > It's being worked on, see this thread on lkml from a few weeks ago: > > C Jan 13 Michael S. Tsir | [PATCH v3 0/4] x86: faster > mb()+documentation tweaks > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 1/4] x86: add cc clobber for > addl > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 2/4] x86: drop a comment left > over from X86_OOSTORE > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 3/4] x86: tweak the comment > about use of wmb for IO > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 4/4] x86: drop mfence in favor > of lock+addl > > The 4th patch changes MFENCE to a LOCK ADDL locked instruction.
Lots of additional chatter here: lkml.kernel.org/r/[email protected] And some useful bits here: lkml.kernel.org/r/[email protected] latest version here: lkml.kernel.org/r/[email protected]
