linux-kernel - Re: [test result] dirty logging without srcu update -- Re: [RFC][PATCH] srcu: Implement call

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Date:	Thu, 02 Feb 2012 19:21:45 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	Avi Kivity <avi@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>, paulmck@...ux.vnet.ibm.com,
	Oleg Nesterov <oleg@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [test result] dirty logging without srcu update -- Re: [RFC][PATCH]
 srcu: Implement call_srcu()

(2012/02/02 19:10), Avi Kivity wrote:

>>
>> =========================================================
>> # 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.

Yes.

I used "unsigned int" just because I wanted to use the current
atomic_clear_mask() as is.

We need to implement atomic_clear_mask_long() or use ...



>
> 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.

Thanks,

I can prepare the official patch series then, of course with more test.


	Takuya

>
> 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.
>
>
>

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