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SubjectRe: [test result] dirty logging without srcu update -- Re: [RFC][PATCH] srcu: Implement call_srcu()
On 02/02/2012 07:46 AM, Takuya Yoshikawa wrote:
> Avi Kivity <avi@redhat.com> wrote:
>
> > >> That'll be great, numbers are better than speculation.
> > >>
> > >
> > >
> > > Yes, I already have some good numbers to show (and some patches).
> >
> > Looking forward.
>
> I made a patch to see if Avi's suggestion of getting rid of srcu
> update for dirty logging is practical; tested with my unit-test.
>
> (I used a function to write protect a range of pages using rmap,
> which is itself useful for optimizing the current code.)
>
> 1. test result
>
> on 32bit host (core i3 box) // just for the unit-test ...
> slot size: 256K pages (1GB memory)
>
>
> Measured by dirty-log-perf (executed only once for each case)
>
> Note: dirty pages are completely distributed
> (no locality: worst case for my patch?)
>
> =========================================================
> # of dirty pages: kvm.git (ns), with this patch (ns)
> 1: 102,077 ns 10,105 ns
> 2: 47,197 ns 9,395 ns
> 4: 43,563 ns 9,938 ns
> 8: 41,239 ns 10,618 ns
> 16: 42,988 ns 12,299 ns
> 32: 45,503 ns 14,298 ns
> 64: 50,915 ns 19,895 ns
> 128: 61,087 ns 29,260 ns
> 256: 81,007 ns 49,023 ns
> 512: 132,776 ns 86,670 ns
> 1024: 939,299 ns 131,496 ns
> 2048: 992,209 ns 250,429 ns
> 4096: 891,809 ns 479,280 ns
> 8192: 1,027,280 ns 906,971 ns
> (until now pretty good)
>
> (ah, for every 32-bit atomic clear mask ...)
> 16384: 1,270,972 ns 6,661,741 ns // 1 1 1 ... 1
> 32768: 1,581,335 ns 9,673,985 ns // ...
> 65536: 2,161,604 ns 11,466,134 ns // ...
> 131072: 3,253,027 ns 13,412,954 ns // ...
> 262144: 5,663,002 ns 16,309,924 ns // 31 31 31 ... 31
> =========================================================

On a 64-bit host, this will be twice as fast. Or if we use cmpxchg16b,
and there are no surprises, four times as fast. It will still be slower
than the original, but by a smaller margin.

> According to a 2005 usenix paper, WWS with a 8sec window was
> about 50,000 pages for a high dirtying rate program.
>
> Taking into acount of another possible gains from the WWS locality
> of real workloads, these numbers are not so bad IMO.

I agree.

>
> Furthermore the code has been made for initial test only and I did
> not do any optimization: I know what I should try.
>
> So this seems worth more testing.
>
>
> The new code also makes it possible to do find-grained get dirty log.
> Live migration can be done like this ??? (not sure yet):
>
> until the dirty rate becomes enough low
> get dirty log for the first 32K pages (partial return is OK)
> while sending
> get dirty log for the next 32K pages (partial return is OK)
> while sending
> ...
> get dirty log for the last 32K pages (partial return is OK)
>
> stop the guest and get dirty log (but no need to write protect now)
> send the remaining pages
>
> New API is needed for this as discussed before!

Yeah. But I think we should switch to srcu-less dirty logs regardless.
Here are you numbers, but normalized by the number of dirty pages.

dirty pages old (ns/page) new (ns/page)
1 102077 10105
2 23599 4698
4 10891 2485
8 5155 1327
16 2687 769
32 1422 447
64 796 311
128 477 229
256 316 191
512 259 169
1024 917 128
2048 484 122
4096 218 117
8192 125 111
16384 78 407
32768 48 295
65536 33 175
131072 25 102
262144 22 62


Your worst case, when considering a reasonable number of dirty pages, is
407ns/page, which is still lower than what userspace will actually do to
process the page, so it's reasonable. The old method is often a lot
worse than your worst case, by this metric.



--
error compiling committee.c: too many arguments to function



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