On Fri, May 08, 2009 at 08:43:40AM -0400, Gregory Haskins wrote: > The problem is the exit time in of itself isnt all that interesting to > me. What I am interested in measuring is how long it takes KVM to > process the request and realize that I want to execute function "X". > Ultimately that is what matters in terms of execution latency and is > thus the more interesting data. I think the exit time is possibly an > interesting 5th data point, but its more of a side-bar IMO. In any > case, I suspect that both exits will be approximately the same at the > VT/SVM level. > > OTOH: If there is a patch out there to improve KVMs code (say > specifically the PIO handling logic), that is fair-game here and we > should benchmark it. For instance, if you have ideas on ways to improve > the find_pio_dev performance, etc.... <guess mode on> One easy thing to try is to cache the last successful lookup on a pointer, to improve patterns where there's "device locality" (like nullio test). <guess mode off> > One item may be to replace the kvm->lock on the bus scan with an RCU > or something.... (though PIOs are very frequent and the constant > re-entry to an an RCU read-side CS may effectively cause a perpetual > grace-period and may be too prohibitive). CC'ing pmck. Yes, locking improvements are needed there badly (think for eg the cache bouncing of kvm->lock _and_ bouncing of kvm->slots_lock on 4-way SMP guests). > FWIW: the PIOoHCs were about 140ns slower than pure HC, so some of that > 140 can possibly be recouped. I currently suspect the lock acquisition > in the iobus-scan is the bulk of that time, but that is admittedly a > guess. The remaining 200-250ns is elsewhere in the PIO decode. vmcs_read is significantly expensive (http://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/msg00840.html, likely that my measurements were foobar, Avi mentioned 50 cycles for vmcs_write). See for eg how vmx.c reads VM_EXIT_INTR_INFO twice on every exit. Also this one looks pretty bad for a 32-bit PAE guest (and you can get away with the unconditional GUEST_CR3 read too). /* Access CR3 don't cause VMExit in paging mode, so we need * to sync with guest real CR3. */ if (enable_ept && is_paging(vcpu)) { vcpu->arch.cr3 = vmcs_readl(GUEST_CR3); ept_load_pdptrs(vcpu); } -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
