Re: [PATCH 0/1] cpufreq: pcc-cpufreq: Re-introduce deadband effect to reduce number of frequency changes

[Andreas Herrmann]

On Fri, Aug 19, 2016 at 02:18:14PM +0200, Andreas Herrmann wrote:
> Hello,
> 
> I've observed performance degradation with different workloads on HP
> ProLiant systems that use pcc-cpufreq driver between older and more
> recent kernels. Bisection showed that commit 6393d6a102 (cpufreq:
> ondemand: Eliminate the deadband effect) caused a significant
> performance drop. This patch was introduced in v3.17.
> 
> I am not too familiar with the PCC stuff but I think that elimination
> of the deadband effect causes a significant increase of requested
> frequency changes which in turn will have to be served by pcc-cpufreq
> and slow down corresponding systems significantly.
> 
> AFAIK there is no frequency table for this driver, instead the driver
> just asks firmware to set any requested frequency for a CPU (if its in
> the min-max-range of allowed frequencies). Thus I think the
> probability that a requested target frequency is matching the current
> frequency of a CPU is lower in comparison to drivers that use a
> fixed set of frequencies.
> 
> This is with two exceptions:
> 
> (1) when a CPU is under full load -- maximum frequency already set and
>     maximum frequency is requested.
> 
> (2) CPU has just "minor load" -- minimum frequency already set and
>     minimum frequency is requested.
> 
> I think what commit 6393d6a102 caused is, that case (2) occurs only if
> the CPU is fully idle. Whereas before commit 6393d6a102 this case
> occurred in all situations when
> 
> 	load < freq_min/freq_max * 100
> 
> I suggest to introduce the old behaviour (re-introduce the deadband effect)
> for pcc-cpufreq with the patch that follows.
> 
> Here are typical numbers from kernel compilation tests with varying
> number of compile jobs:
> 
>                      v4.8.0-rc2               4.8.0-rc2-pcc-cpufreq-deadband
>  # of jobst   user     sys   elapsed   CPU     user     sys   elapsed   CPU
>        2     440.39  116.49  4:33.35   203%   404.85  109.10  4:10.35   205%
>        4     436.87  133.39  2:22.88   399%   381.83  128.00  2:06.84   401%
>        8     475.49  157.68  1:22.24   769%   344.36  149.08  1:04.29   767%
>       16     620.69  188.33  0:54.74  1477%   374.60  157.40  0:36.76  1447%
>       32     815.79  209.58  0:37.22  2754%   490.46  160.22  0:24.87  2616%
>       64     394.13   60.55  0:13.54  3355%   386.54   60.33  0:12.79  3493%
>      120     398.24   61.55  0:14.60  3148%   390.44   61.19  0:13.07  3453%
> 
> As expected under full load (64, 120 jobs) performance is "almost"
> comparable. But with partial load (esp. 32 jobs) length of kernel
> build is just 2/3 with patched kernel in comparison to current
> mainline. Similar behaviour is observable since introduction of commit
> 6393d6a102 (ie. since v3.17).
> 
> Numbers are from an HP ProLiant DL580 Gen8 system:
>  	- Intel(R) Xeon(R) CPU E7-4890 v2 @ 2.80GHz
>         - 60 CPUs, 128GB RAM
> 
> PCC info of this system
> 
>   PCCH header (virtual) addr: 0xffffc9000d840000
>   PCCH header is at physical address: 0x7ac59000, signature: 0x24504343,
>     length: 64 bytes, major: 1, minor: 0, supported features: 0x1,
>     command field: 0x1, status field: 0x0, nominal latency: 300 us
>     min time between commands: 50000 us, max time between commands: 1000000 us,
>     nominal CPU frequency: 2800 MHz, minimum CPU frequency: 150 MHz,
>     minimum CPU frequency without throttling: 1200 MHz
> 
> Kernel message when driver loads:
>   pcc-cpufreq: (v1.10.00) driver loaded with frequency limits: 1200 MHz, 2800 MHz
> 
> My patch adds a debug message, which on this system looks like
>   pcc-cpufreq: setting deadband limit to 1885000 kHz

BTW, I've forgotten to point out that my proposed change does not
fully restore the old behaviour (before v3.17).

The difference is that the combination of commit 6393d6a102 and my
change still maps frequencies previously requested between
[min_freq,max_freq] to the range [limit,max_freq].

So on above system pcc-cpufreq will only request freqencies that
are either 1200 MHz or between [1885 MHz, 2800 MHz].


Andreas