On Fri, 2013-06-07 at 11:45 +0530, Raghavendra K T wrote: > On 06/03/2013 11:51 AM, Raghavendra K T wrote: > > On 06/03/2013 07:10 AM, Raghavendra K T wrote: > >> On 06/02/2013 09:50 PM, Jiannan Ouyang wrote: > >>> On Sun, Jun 2, 2013 at 1:07 AM, Gleb Natapov <gleb@xxxxxxxxxx> wrote: > >>> > >>>> High level question here. We have a big hope for "Preemptable Ticket > >>>> Spinlock" patch series by Jiannan Ouyang to solve most, if not all, > >>>> ticketing spinlocks in overcommit scenarios problem without need for > >>>> PV. > >>>> So how this patch series compares with his patches on PLE enabled > >>>> processors? > >>>> > >>> > >>> No experiment results yet. > >>> > >>> An error is reported on a 20 core VM. I'm during an internship > >>> relocation, and will start work on it next week. > >> > >> Preemptable spinlocks' testing update: > >> I hit the same softlockup problem while testing on 32 core machine with > >> 32 guest vcpus that Andrew had reported. > >> > >> After that i started tuning TIMEOUT_UNIT, and when I went till (1<<8), > >> things seemed to be manageable for undercommit cases. > >> But I still see degradation for undercommit w.r.t baseline itself on 32 > >> core machine (after tuning). > >> > >> (37.5% degradation w.r.t base line). > >> I can give the full report after the all tests complete. > >> > >> For over-commit cases, I again started hitting softlockups (and > >> degradation is worse). But as I said in the preemptable thread, the > >> concept of preemptable locks looks promising (though I am still not a > >> fan of embedded TIMEOUT mechanism) > >> > >> Here is my opinion of TODOs for preemptable locks to make it better ( I > >> think I need to paste in the preemptable thread also) > >> > >> 1. Current TIMEOUT UNIT seem to be on higher side and also it does not > >> scale well with large guests and also overcommit. we need to have a > >> sort of adaptive mechanism and better is sort of different TIMEOUT_UNITS > >> for different types of lock too. The hashing mechanism that was used in > >> Rik's spinlock backoff series fits better probably. > >> > >> 2. I do not think TIMEOUT_UNIT itself would work great when we have a > >> big queue (for large guests / overcommits) for lock. > >> one way is to add a PV hook that does yield hypercall immediately for > >> the waiters above some THRESHOLD so that they don't burn the CPU. > >> ( I can do POC to check if that idea works in improving situation > >> at some later point of time) > >> > > > > Preemptable-lock results from my run with 2^8 TIMEOUT: > > > > +-----------+-----------+-----------+------------+-----------+ > > ebizzy (records/sec) higher is better > > +-----------+-----------+-----------+------------+-----------+ > > base stdev patched stdev %improvement > > +-----------+-----------+-----------+------------+-----------+ > > 1x 5574.9000 237.4997 3484.2000 113.4449 -37.50202 > > 2x 2741.5000 561.3090 351.5000 140.5420 -87.17855 > > 3x 2146.2500 216.7718 194.8333 85.0303 -90.92215 > > 4x 1663.0000 141.9235 101.0000 57.7853 -93.92664 > > +-----------+-----------+-----------+------------+-----------+ > > +-----------+-----------+-----------+------------+-----------+ > > dbench (Throughput) higher is better > > +-----------+-----------+-----------+------------+-----------+ > > base stdev patched stdev %improvement > > +-----------+-----------+-----------+------------+-----------+ > > 1x 14111.5600 754.4525 3930.1602 2547.2369 -72.14936 > > 2x 2481.6270 71.2665 181.1816 89.5368 -92.69908 > > 3x 1510.2483 31.8634 104.7243 53.2470 -93.06576 > > 4x 1029.4875 16.9166 72.3738 38.2432 -92.96992 > > +-----------+-----------+-----------+------------+-----------+ > > > > Note we can not trust on overcommit results because of softlock-ups > > > > Hi, I tried > (1) TIMEOUT=(2^7) > > (2) having yield hypercall that uses kvm_vcpu_on_spin() to do directed > yield to other vCPUs. > > Now I do not see any soft-lockup in overcommit cases and results are > better now (except ebizzy 1x). and for dbench I see now it is closer to > base and even improvement in 4x > > +-----------+-----------+-----------+------------+-----------+ > ebizzy (records/sec) higher is better > +-----------+-----------+-----------+------------+-----------+ > base stdev patched stdev %improvement > +-----------+-----------+-----------+------------+-----------+ > 5574.9000 237.4997 523.7000 1.4181 -90.60611 > 2741.5000 561.3090 597.8000 34.9755 -78.19442 > 2146.2500 216.7718 902.6667 82.4228 -57.94215 > 1663.0000 141.9235 1245.0000 67.2989 -25.13530 > +-----------+-----------+-----------+------------+-----------+ > +-----------+-----------+-----------+------------+-----------+ > dbench (Throughput) higher is better > +-----------+-----------+-----------+------------+-----------+ > base stdev patched stdev %improvement > +-----------+-----------+-----------+------------+-----------+ > 14111.5600 754.4525 884.9051 24.4723 -93.72922 > 2481.6270 71.2665 2383.5700 333.2435 -3.95132 > 1510.2483 31.8634 1477.7358 50.5126 -2.15279 > 1029.4875 16.9166 1075.9225 13.9911 4.51050 > +-----------+-----------+-----------+------------+-----------+ > > > IMO hash based timeout is worth a try further. > I think little more tuning will get more better results. The problem I see (especially for dbench) is that we are still way off what I would consider the goal. IMO, 2x over-commit result should be a bit lower than 50% (to account for switching overhead and less cache warmth). We are at about 17.5% for 2x. I am thinking we need a completely different approach to get there, but of course I do not know what that is yet :) I am testing your patches now and hopefully with some analysis data we can better understand what's going on. > > Jiannan, When you start working on this, I can also help > to get best of preemptable lock idea if you wish and share > the patches I tried. -Andrew Theurer -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
