win7 bad i/o performance, high insn_emulation and exists

[Peter Lieven <pl@dlh.net>]

Hi,

I came a across an issue with a Windows 7 (32-bit) as well as with a 
Windows 2008 R2 (64-bit) guest.

If I transfer a file from the VM via CIFS or FTP to a remote machine,
i get very poor read performance (around 13MB/s). The VM peaks at 100%
cpu and I see a lot of insn_emulations and all kinds of exists in kvm_stat

efer_reload                    0         0
exits                    2260976     79620
fpu_reload                  6197        11
halt_exits                114734      5011
halt_wakeup               111195      4876
host_state_reload        1499659     60962
hypercalls                     0         0
insn_emulation           1577325     58488
insn_emulation_fail            0         0
invlpg                         0         0
io_exits                  943949     40249
irq_exits                 108679      5434
irq_injections            236545     10788
irq_window                  7606       246
largepages                   672         5
mmio_exits                460020     16082
mmu_cache_miss               119         0
mmu_flooded                    0         0
mmu_pde_zapped                 0         0
mmu_pte_updated                0         0
mmu_pte_write              13474         9
mmu_recycled                   0         0
mmu_shadow_zapped            141         0
mmu_unsync                     0         0
nmi_injections                 0         0
nmi_window                     0         0
pf_fixed                   22803        35
pf_guest                       0         0
remote_tlb_flush             239         2
request_irq                    0         0
signal_exits                   0         0
tlb_flush                  20933         0

If I run the same VM with a Ubuntu 10.04.4 guest I get around 60MB/s 
throughput. The kvm_stats look a lot more sane.

efer_reload                    0         0
exits                    6132004     17931
fpu_reload                 19863         3
halt_exits                264961      3083
halt_wakeup               236468      2959
host_state_reload        1104468      3104
hypercalls                     0         0
insn_emulation           1417443      7518
insn_emulation_fail            0         0
invlpg                         0         0
io_exits                  869380      2795
irq_exits                 253501      2362
irq_injections            616967      6804
irq_window                201186      2161
largepages                  1019         0
mmio_exits                205268         0
mmu_cache_miss               192         0
mmu_flooded                    0         0
mmu_pde_zapped                 0         0
mmu_pte_updated                0         0
mmu_pte_write            7440546         0
mmu_recycled                   0         0
mmu_shadow_zapped            259         0
mmu_unsync                     0         0
nmi_injections                 0         0
nmi_window                     0         0
pf_fixed                   38529        30
pf_guest                       0         0
remote_tlb_flush             761         1
request_irq                    0         0
signal_exits                   0         0
tlb_flush                      0         0

I use virtio-net (with vhost-net) and virtio-blk. I tried disabling hpet 
(which basically illiminated the mmio_exits, but does not increase
performance) and also commit (39a7a362e16bb27e98738d63f24d1ab5811e26a8
) - no improvement.

My commandline:
/usr/bin/qemu-kvm-1.0 -netdev 
type=tap,id=guest8,script=no,downscript=no,ifname=tap0,vhost=on -device 
virtio-net-pci,netdev=guest8,mac=52:54:00:ff:00:d3 -drive 
format=host_device,file=/dev/mapper/iqn.2001-05.com.equallogic:0-8a0906-eeef4e007-a8a9f3818674f2fc-lieven-windows7-vc-r80788,if=virtio,cache=none,aio=native 
-m 2048 -smp 2 -monitor tcp:0:4001,server,nowait -vnc :1 -name 
lieven-win7-vc -boot order=dc,menu=off -k de -pidfile 
/var/run/qemu/vm-187.pid -mem-path /hugepages -mem-prealloc -cpu host 
-rtc base=localtime -vga std -usb -usbdevice tablet -no-hpet

What further information is needed to debug this further?

Thanks,
Peter