Hi Marcelo: mmu_recycled is always 0 for this guest -- even after almost 4 hours of uptime.
Here is a kvm_stat sample where guest time was very high and qemu had 2 processors at 100% on the host. I removed counters where both columns have 0 value for brevity. exits 45937979 758051 fpu_reload 1416831 87 halt_exits 112911 0 halt_wakeup 31771 0 host_state_reload 2068602 263 insn_emulation 21601480 365493 io_exits 1827374 2705 irq_exits 8934818 285196 mmio_exits 421674 147 mmu_cache_miss 4817689 93680 mmu_flooded 4815273 93680 mmu_pde_zapped 51344 0 mmu_prefetch 4817625 93680 mmu_pte_updated 14803298 270104 mmu_pte_write 19859863 363785 mmu_shadow_zapped 4832106 93679 pf_fixed 32184355 468398 pf_guest 264138 0 remote_tlb_flush 10697762 280522 tlb_flush 10301338 176424 (NOTE: This is for a *5* second sample interval instead of 1 to allow me to capture the data). Here's a sample when the guest is "well-behaved" (system time <10%, though ): exits 51502194 97453 fpu_reload 1421736 227 halt_exits 138361 1927 halt_wakeup 33047 117 host_state_reload 2110190 3740 insn_emulation 24367441 47260 io_exits 1874075 2576 irq_exits 10224702 13333 mmio_exits 435154 1726 mmu_cache_miss 5414097 11258 mmu_flooded 5411548 11243 mmu_pde_zapped 52851 44 mmu_prefetch 5414031 11258 mmu_pte_updated 16854686 29901 mmu_pte_write 22526765 42285 mmu_shadow_zapped 5430025 11313 pf_fixed 36144578 67666 pf_guest 282794 430 remote_tlb_flush 12126268 14619 tlb_flush 11753162 21460 There is definitely a strong correlation between the mmu counters and high system times in the guest. I am still trying to find out what in the guest is stimulating it when running on RHEL3; I do not see this same behavior for an equivalent setup running on RHEL4. By the way I added an mmu_prefetch stat in prefetch_page() to count the number of times the for() loop is hit with PTTYPE == 64; ie., number of times paging64_prefetch_page() is invoked. (I wanted an explicit counter for this loop, though the info seems to duplicate other entries.) That counter is listed above. As I mentioned in a prior post when kscand kicks in the change in mmu_prefetch counter is at 20,000+/sec, with each trip through that function taking 45k+ cycles. kscand is an instigator shortly after boot, however, kscand is *not* the culprit once the system has been up for 30-45 minutes. I have started instrumenting the RHEL3U8 kernel and for the load I am running kscand does not walk the active lists very often once the system is up. So, to dig deeper on what in the guest is stimulating the mmu I collected kvmtrace data for about a 2 minute time interval which caught about a 30-second period where guest system time was steady in the 25-30% range. Summarizing the number of times a RIP appears in an VMEXIT shows the following high runners: count RIP RHEL3-symbol 82549 0xc0140e42 follow_page [kernel] c0140d90 offset b2 42532 0xc0144760 handle_mm_fault [kernel] c01446d0 offset 90 36826 0xc013da4a futex_wait [kernel] c013d870 offset 1da 29987 0xc0145cd0 zap_pte_range [kernel] c0145c10 offset c0 27451 0xc0144018 do_no_page [kernel] c0143e20 offset 1f8 (halt entry removed the list since that is the ideal scenario for an exit). So the RIP correlates to follow_page() for a large percentage of the VMEXITs. I wrote an awk script to summarize (histogram style) the TSC cycles between VMEXIT and VMENTRY for an address. For the first rip, 0xc0140e42, 82,271 times (ie., almost 100% of the time) the trace shows a delta between 50k and 100k cycles between the VMEXIT and the subsequent VMENTRY. Similarly for the second one, 0xc0144760, 42403 times (again almost 100% of the occurrences) the trace shows a delta between 50k and 100k cycles between VMEXIT and VMENTRY. These seems to correlate with the prefetch_page function in kvm, though I am not 100% positive on that. I am now investigating the kernel paths leading to those functions. Any insights would definitely be appreciated. david Marcelo Tosatti wrote: > On Fri, Apr 25, 2008 at 11:33:18AM -0600, David S. Ahern wrote: >> Most of the cycles (~80% of that 54k+) are spent in paging64_prefetch_page(): >> >> for (i = 0; i < PT64_ENT_PER_PAGE; ++i) { >> gpa_t pte_gpa = gfn_to_gpa(sp->gfn); >> pte_gpa += (i+offset) * sizeof(pt_element_t); >> >> r = kvm_read_guest_atomic(vcpu->kvm, pte_gpa, &pt, >> sizeof(pt_element_t)); >> if (r || is_present_pte(pt)) >> sp->spt[i] = shadow_trap_nonpresent_pte; >> else >> sp->spt[i] = shadow_notrap_nonpresent_pte; >> } >> >> This loop is run 512 times and takes a total of ~45k cycles, or ~88 cycles >> per >> loop. >> >> This function gets run >20,000/sec during some of the kscand loops. > > Hi David, > > Do you see the mmu_recycled counter increase? > ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. 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