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Date: Thu, 10 Sep 2009 08:08:24 +0200
From: Ingo Molnar <mingo@...e.hu>
To: Nikos Chantziaras <realnc@...or.de>
Cc: Jens Axboe <jens.axboe@...cle.com>, Mike Galbraith <efault@....de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Con Kolivas <kernel@...ivas.org>, linux-kernel@...r.kernel.org
Subject: Re: BFS vs. mainline scheduler benchmarks and measurements
* Nikos Chantziaras <realnc@...or.de> wrote:
> On 09/09/2009 09:04 PM, Ingo Molnar wrote:
>> [...]
>> * Jens Axboe<jens.axboe@...cle.com> wrote:
>>
>>> On Wed, Sep 09 2009, Jens Axboe wrote:
>>> [...]
>>> BFS210 runs on the laptop (dual core intel core duo). With make -j4
>>> running, I clock the following latt -c8 'sleep 10' latencies:
>>>
>>> -rc9
>>>
>>> Max 17895 usec
>>> Avg 8028 usec
>>> Stdev 5948 usec
>>> Stdev mean 405 usec
>>>
>>> Max 17896 usec
>>> Avg 4951 usec
>>> Stdev 6278 usec
>>> Stdev mean 427 usec
>>>
>>> Max 17885 usec
>>> Avg 5526 usec
>>> Stdev 6819 usec
>>> Stdev mean 464 usec
>>>
>>> -rc9 + mike
>>>
>>> Max 6061 usec
>>> Avg 3797 usec
>>> Stdev 1726 usec
>>> Stdev mean 117 usec
>>>
>>> Max 5122 usec
>>> Avg 3958 usec
>>> Stdev 1697 usec
>>> Stdev mean 115 usec
>>>
>>> Max 6691 usec
>>> Avg 2130 usec
>>> Stdev 2165 usec
>>> Stdev mean 147 usec
>>
>> At least in my tests these latencies were mainly due to a bug in
>> latt.c - i've attached the fixed version.
>>
>> The other reason was wakeup batching. If you do this:
>>
>> echo 0> /proc/sys/kernel/sched_wakeup_granularity_ns
>>
>> ... then you can switch on insta-wakeups on -tip too.
>>
>> With a dual-core box and a make -j4 background job running, on
>> latest -tip i get the following latencies:
>>
>> $ ./latt -c8 sleep 30
>> Entries: 656 (clients=8)
>>
>> Averages:
>> ------------------------------
>> Max 158 usec
>> Avg 12 usec
>> Stdev 10 usec
>
> With your version of latt.c, I get these results with 2.6-tip vs
> 2.6.31-rc9-bfs:
>
>
> (mainline)
> Averages:
> ------------------------------
> Max 50 usec
> Avg 12 usec
> Stdev 3 usec
>
>
> (BFS)
> Averages:
> ------------------------------
> Max 474 usec
> Avg 11 usec
> Stdev 16 usec
>
> However, the interactivity problems still remain. Does that mean
> it's not a latency issue?
It means that Jens's test-app, which demonstrated and helped us fix
the issue for him does not help us fix it for you just yet.
The "fluidity problem" you described might not be a classic latency
issue per se (which latt.c measures), but a timeslicing / CPU time
distribution problem.
A slight shift in CPU time allocation can change the flow of tasks
to result in a 'choppier' system.
Have you tried, in addition of the granularity tweaks you've done,
to renice mplayer either up or down? (or compiz and Xorg for that
matter)
I'm not necessarily suggesting this as a 'real' solution (we really
prefer kernels that just get it right) - but it's an additional
parameter dimension along which you can tweak CPU time distribution
on your box.
Here's the general rule of thumb: mine one nice level gives plus 5%
CPU time to a task and takes away 5% CPU time from another task -
i.e. shifts the CPU allocation by 10%.
( this is modified by all sorts of dynamic conditions: by the number
of tasks running and their wakeup patters so not a rule cast into
stone - but still a good ballpark figure for CPU intense tasks. )
Btw., i've read your descriptions about what you've tuned so far -
have you seen/checked the wakeup_granularity tunable as well?
Setting that to 0 will change the general balance of how CPU time is
allocated between tasks too.
There's also a whole bunch of scheduler features you can turn on/off
individually via /debug/sched_features. For example, to turn off
NEW_FAIR_SLEEPERS, you can do:
# cat /debug/sched_features
NEW_FAIR_SLEEPERS NO_NORMALIZED_SLEEPER ADAPTIVE_GRAN WAKEUP_PREEMPT
START_DEBIT AFFINE_WAKEUPS CACHE_HOT_BUDDY SYNC_WAKEUPS NO_HRTICK
NO_DOUBLE_TICK ASYM_GRAN LB_BIAS LB_WAKEUP_UPDATE ASYM_EFF_LOAD
NO_WAKEUP_OVERLAP LAST_BUDDY OWNER_SPIN
# echo NO_NEW_FAIR_SLEEPERS > /debug/sched_features
Btw., NO_NEW_FAIR_SLEEPERS is something that will turn the scheduler
into a more classic fair scheduler (like BFS is too).
NO_START_DEBIT might be another thing that improves (or worsens :-/)
make -j type of kernel build workloads.
Note, these flags are all runtime, the new settings take effect
almost immediately (and at the latest it takes effect when a task
has started up) and safe to do runtime.
It basically gives us 32768 pluggable schedulers each with a
slightly separate algorithm - each setting in essence creates a new
scheduler. (this mechanism is how we introduce new scheduler
features and allow their debugging / regression-testing.)
(okay, almost, so beware: turning on HRTICK might lock up your
system.)
Plus, yet another dimension of tuning on SMP systems (such as
dual-core) are the sched-domains tunable. There's a whole world of
tuning in that area and BFS essentially implements a very agressive
'always balance to other CPUs' policy.
I've attached my sched-tune-domains script which helps tune these
parameters.
For example on a testbox of mine it outputs:
usage: tune-sched-domains <val>
{cpu0/domain0:SIBLING} SD flag: 239
+ 1: SD_LOAD_BALANCE: Do load balancing on this domain
+ 2: SD_BALANCE_NEWIDLE: Balance when about to become idle
+ 4: SD_BALANCE_EXEC: Balance on exec
+ 8: SD_BALANCE_FORK: Balance on fork, clone
- 16: SD_WAKE_IDLE: Wake to idle CPU on task wakeup
+ 32: SD_WAKE_AFFINE: Wake task to waking CPU
+ 64: SD_WAKE_BALANCE: Perform balancing at task wakeup
+ 128: SD_SHARE_CPUPOWER: Domain members share cpu power
- 256: SD_POWERSAVINGS_BALANCE: Balance for power savings
- 512: SD_SHARE_PKG_RESOURCES: Domain members share cpu pkg resources
-1024: SD_SERIALIZE: Only a single load balancing instance
-2048: SD_WAKE_IDLE_FAR: Gain latency sacrificing cache hit
-4096: SD_PREFER_SIBLING: Prefer to place tasks in a sibling domain
{cpu0/domain1:MC} SD flag: 4735
+ 1: SD_LOAD_BALANCE: Do load balancing on this domain
+ 2: SD_BALANCE_NEWIDLE: Balance when about to become idle
+ 4: SD_BALANCE_EXEC: Balance on exec
+ 8: SD_BALANCE_FORK: Balance on fork, clone
+ 16: SD_WAKE_IDLE: Wake to idle CPU on task wakeup
+ 32: SD_WAKE_AFFINE: Wake task to waking CPU
+ 64: SD_WAKE_BALANCE: Perform balancing at task wakeup
- 128: SD_SHARE_CPUPOWER: Domain members share cpu power
- 256: SD_POWERSAVINGS_BALANCE: Balance for power savings
+ 512: SD_SHARE_PKG_RESOURCES: Domain members share cpu pkg resources
-1024: SD_SERIALIZE: Only a single load balancing instance
-2048: SD_WAKE_IDLE_FAR: Gain latency sacrificing cache hit
+4096: SD_PREFER_SIBLING: Prefer to place tasks in a sibling domain
{cpu0/domain2:NODE} SD flag: 3183
+ 1: SD_LOAD_BALANCE: Do load balancing on this domain
+ 2: SD_BALANCE_NEWIDLE: Balance when about to become idle
+ 4: SD_BALANCE_EXEC: Balance on exec
+ 8: SD_BALANCE_FORK: Balance on fork, clone
- 16: SD_WAKE_IDLE: Wake to idle CPU on task wakeup
+ 32: SD_WAKE_AFFINE: Wake task to waking CPU
+ 64: SD_WAKE_BALANCE: Perform balancing at task wakeup
- 128: SD_SHARE_CPUPOWER: Domain members share cpu power
- 256: SD_POWERSAVINGS_BALANCE: Balance for power savings
- 512: SD_SHARE_PKG_RESOURCES: Domain members share cpu pkg resources
+1024: SD_SERIALIZE: Only a single load balancing instance
+2048: SD_WAKE_IDLE_FAR: Gain latency sacrificing cache hit
-4096: SD_PREFER_SIBLING: Prefer to place tasks in a sibling domain
The way i can turn on say SD_WAKE_IDLE for the NODE domain is to:
tune-sched-domains 239 4735 $((3183+16))
( This is a pretty stone-age script i admit ;-)
Thanks for all your testing so far,
Ingo
View attachment "tune-sched-domains" of type "text/plain" (2153 bytes)
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