On Tue, Jan 12, 2016 at 12:59:58PM -0800, Andy Lutomirski wrote: > On Tue, Jan 12, 2016 at 12:54 PM, Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Tue, Jan 12, 2016 at 12:30 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: > >> > >> I recall reading somewhere that lock addl $0, 32(%rsp) or so (maybe even 64) > >> was better because it avoided stomping on very-likely-to-be-hot write > >> buffers. > > > > I suspect it could go either way. You want a small constant (for the > > isntruction size), but any small constant is likely to be within the > > current stack frame anyway. I don't think 0(%rsp) is particularly > > likely to have a spill on it right then and there, but who knows.. > > > > And 64(%rsp) is possibly going to be cold in the L1 cache, especially > > if it's just after a deep function call. Which it might be. So it > > might work the other way. > > > > So my guess would be that you wouldn't be able to measure the > > difference. It might be there, but probably too small to really see in > > any noise. > > > > But numbers talk, bullshit walks. It would be interesting to be proven wrong. > > Here's an article with numbers: > > http://shipilev.net/blog/2014/on-the-fence-with-dependencies/ > > I think they're suggesting using a negative offset, which is safe as > long as it doesn't page fault, even though we have the redzone > disabled. > > --Andy OK so I'll have to tweak the test to put something on stack to measure the difference: my test tweaks a global variable instead. I'll try that by tomorrow. I couldn't measure any difference between mfence and lock+addl except in a micro-benchmark, but hey since we are tweaking this, let's do the optimal thing. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
