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

[Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>]

>>
>> get.org get.opt switch.opt
>>
>> slots[7].len2768 278379 66398 64024
>> slots[8].len2768 181246 270 160
>> slots[7].len2768 263961 64673 64494
>> slots[8].len2768 181655 265 160
>> slots[7].len2768 263736 64701 64610
>> slots[8].len2768 182785 267 160
>> slots[7].len2768 260925 65360 65042
>> slots[8].len2768 182579 264 160
>> slots[7].len2768 267823 65915 65682
>> slots[8].len2768 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).

Sorry but no, and I agree with your guess.
Anyway, I want to do some profiling to confirm this guess.


BTW, If we only think about performance improvement of time, optimized
get(get.opt) may be enough at this stage.

But if we consider the future expansion like using user allocated bitmaps,
new API's introduced for switch.opt won't become waste, I think, because we
need a structure to get and export bitmap addresses.


>>
>> 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?

OK, I'll do!

>