On Fri, Jan 29, 2016 at 6:26 AM, Borislav Petkov <b...@alien8.de> wrote: > On Mon, Jan 25, 2016 at 10:37:44AM -0800, Andy Lutomirski wrote: >> On my Skylake laptop, INVPCID function 2 (flush absolutely >> everything) takes about 376ns, whereas saving flags, twiddling >> CR4.PGE to flush global mappings, and restoring flags takes about >> 539ns. > > FWIW, I ran your microbenchmark on the IVB laptop I have here 3 times > and some of the numbers from each run are pretty unstable. Not that it > means a whole lot - the thing doesn't have INVPCID support. > > I'm just questioning the microbenchmark and whether we should be rather > doing those measurements with a real benchmark, whatever that means. My > limited experience says that measuring TLB performance is hard. > > ./context_switch_latency 0 thread same > use_xstate = 0 > Using threads > 1: 100000 iters at 2676.2 ns/switch > 2: 100000 iters at 2700.2 ns/switch > 3: 100000 iters at 2656.1 ns/switch > > ./context_switch_latency 0 thread different > use_xstate = 0 > Using threads > 1: 100000 iters at 5174.8 ns/switch > 2: 100000 iters at 5140.5 ns/switch > 3: 100000 iters at 5292.9 ns/switch > > ./context_switch_latency 0 process same > use_xstate = 0 > Using a subprocess > 1: 100000 iters at 2361.2 ns/switch > 2: 100000 iters at 2332.2 ns/switch > 3: 100000 iters at 3436.9 ns/switch > > ./context_switch_latency 0 process different > use_xstate = 0 > Using a subprocess > 1: 100000 iters at 4713.6 ns/switch > 2: 100000 iters at 4957.5 ns/switch > 3: 100000 iters at 5012.2 ns/switch > > ./context_switch_latency 1 thread same > use_xstate = 1 > Using threads > 1: 100000 iters at 2505.6 ns/switch > 2: 100000 iters at 2483.1 ns/switch > 3: 100000 iters at 2479.7 ns/switch > > ./context_switch_latency 1 thread different > use_xstate = 1 > Using threads > 1: 100000 iters at 5245.9 ns/switch > 2: 100000 iters at 5241.1 ns/switch > 3: 100000 iters at 5220.3 ns/switch > > ./context_switch_latency 1 process same > use_xstate = 1 > Using a subprocess > 1: 100000 iters at 2329.8 ns/switch > 2: 100000 iters at 2350.2 ns/switch > 3: 100000 iters at 2500.9 ns/switch > > ./context_switch_latency 1 process different > use_xstate = 1 > Using a subprocess > 1: 100000 iters at 4970.7 ns/switch > 2: 100000 iters at 5034.0 ns/switch > 3: 100000 iters at 4991.6 ns/switch >
I'll fiddle with that benchmark a little bit. Maybe I can make it suck less. If anyone knows a good non-micro benchmark for this, let me know. I refuse to use dbus as my benchmark :) FWIW, I benchmarked cr4 vs invpcid by adding a prctl and calling it in a loop. If Ingo's fpu benchmark thing ever lands, I'll gladly send a patch to add TLB flushes to it. --Andy
