Re: [tbench regression fixes]: digging out smelly deadmen.

Ingo Molnar
2008-10-10T09:15:36Z

hi Evgeniy,

* Evgeniy Polyakov <s0mbre@tservice.net.ru> wrote:

> Hi Peter.
>
> I've enabled kernel hacking option and scheduler debugging and turned
> off hrticks and performance jumped to 382 MB/s:
>
> vanilla 27: 347.222
> no TSO/GSO: 357.331
> no hrticks: 382.983
>
> I use tsc clocksource, also available acpi_pm and jiffies,
> with acpi_pm performance is even lower (I stopped test after it dropped
> below 340 MB/s mark), jiffies do not work at all, looks like sockets
> stuck in time_wait state when this clock source is used, although that
> may be some different issue.
>
> So I think hrticks are guilty, but still not as good as .25 tree without
> mentioned changes (455 MB/s) and .24 (475 MB/s).

i'm glad that you are looking into this! That is an SMP box, right? If
yes then could you try this sched-domains tuning utility i have written
yesterday (incidentally):

http://redhat.com/~mingo/cfs-scheduler/tune-sched-domains

just run it without options to see the current sched-domains options. On
a testsystem i have it displays this:

# tune-sched-domains
usage: tune-sched-domains <val>
current val on cpu0/domain0:
SD flag: 47
+ 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

then could you check what effects it has if you turn off
SD_BALANCE_NEWIDLE? On my box i did it via:

# tune-sched-domains $[47-2]
changed /proc/sys/kernel/sched_domain/cpu0/domain0/flags: 47 => 45
SD flag: 45
+ 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
changed /proc/sys/kernel/sched_domain/cpu0/domain1/flags: 1101 => 45
SD flag: 45
+ 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

and please, when tuning such scheduler bits, could you run latest
tip/master:

http://people.redhat.com/mingo/tip.git/README

and you need to have CONFIG_SCHED_DEBUG=y enabled for the tuning knobs.

so that it's all in sync with upcoming scheduler changes/tunings/fixes.

It will also make it much easier for us to apply any fix patches you
might send :-)

For advanced tuners: you can specify two or more domain flags options as
well on the command line - that will be put into domain1/domain2/etc. I
usually tune these flags via something like:

tune-sched-domains $[1*1+1*2+1*4+1*8+0*16+1*32+1*64]

that makes it easy to set/clear each of the flags.

Ingo