Subject: Re: [RFC][PATCH 0/12] KVM, x86, ppc, asm-generic: moving dirty bitmaps to user space

From: Avi Kivity <[email-masked]>
Date: Mon, 10 May 2010 15:06:47 +0300

On 05/04/2010 03:56 PM, Takuya Yoshikawa wrote:
> [Performance test]
>
> We measured the tsc needed to the ioctl()s for getting dirty logs in
> kernel.
>
> Test environment
>
>    AMD Phenom(tm) 9850 Quad-Core Processor with 8GB memory
>
>
> 1. GUI test (running Ubuntu guest in graphical mode)
>
>    sudo qemu-system-x86_64 -hda dirtylog_test.img -boot c -m 4192 -net ...
>
> We show a relatively stable part to compare how much time is needed
> for the basic parts of dirty log ioctl.
>
>                             get.org   get.opt  switch.opt
>
> slots[7].len=32768          278379     66398     64024
> slots[8].len=32768          181246       270       160
> slots[7].len=32768          263961     64673     64494
> slots[8].len=32768          181655       265       160
> slots[7].len=32768          263736     64701     64610
> slots[8].len=32768          182785       267       160
> slots[7].len=32768          260925     65360     65042
> slots[8].len=32768          182579       264       160
> slots[7].len=32768          267823     65915     65682
> slots[8].len=32768          186350       271       160
>
> At a glance, we know our optimization improved significantly compared
> to the original get dirty log ioctl. This is true for both get.opt and
> switch.opt. This has a really big impact for the personal KVM users who
> drive KVM in GUI mode on their usual PCs.
>
> Next, we notice that switch.opt improved a hundred nano seconds or so for
> these slots. Although this may sound a bit tiny improvement, we can feel
> this as a difference of GUI's responses like mouse reactions.
>    

100 ns... this is a bit on the low side (and if you can measure it 
interactively you have much better reflexes than I).

> To feel the difference, please try GUI on your PC with our patch series!
>    

No doubt get.org -> get.opt is measurable, but get.opt->switch.opt is 
problematic.  Have you tried profiling to see where the time is spent 
(well I can guess, clearing the write access from the sptes).

>
> 2. Live-migration test (4GB guest, write loop with 1GB buf)
>
> We also did a live-migration test.
>
>                             get.org   get.opt  switch.opt
>
> slots[0].len=655360         797383    261144    222181
> slots[1].len=3757047808    2186721   1965244   1842824
> slots[2].len=637534208     1433562   1012723   1031213
> slots[3].len=131072         216858       331       331
> slots[4].len=131072         121635       225       164
> slots[5].len=131072         120863       356       164
> slots[6].len=16777216       121746      1133       156
> slots[7].len=32768          120415       230       278
> slots[8].len=32768          120368       216       149
> slots[0].len=655360         806497    194710    223582
> slots[1].len=3757047808    2142922   1878025   1895369
> slots[2].len=637534208     1386512   1021309   1000345
> slots[3].len=131072         221118       459       296
> slots[4].len=131072         121516       272       166
> slots[5].len=131072         122652       244       173
> slots[6].len=16777216       123226     99185       149
> slots[7].len=32768          121803       457       505
> slots[8].len=32768          121586       216       155
> slots[0].len=655360         766113    211317    213179
> slots[1].len=3757047808    2155662   1974790   1842361
> slots[2].len=637534208     1481411   1020004   1031352
> slots[3].len=131072         223100       351       295
> slots[4].len=131072         122982       436       164
> slots[5].len=131072         122100       300       503
> slots[6].len=16777216       123653       779       151
> slots[7].len=32768          122617       284       157
> slots[8].len=32768          122737       253       149
>
> For slots other than 0,1,2 we can see the similar improvement.
>
> Considering the fact that switch.opt does not depend on the bitmap length
> except for kvm_mmu_slot_remove_write_access(), this is the cause of some
> usec to msec time consumption: there might be some context switches.
>
> But note that this was done with the workload which dirtied the memory
> endlessly during the live-migration.
>
> In usual workload, the number of dirty pages varies a lot for each iteration
> and we should gain really a lot for relatively clean cases.
>    

Can you post such a test, for an idle large guest?

-- 
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