Re: Low Latency vs. Real Time Kernel - actual latencies ?

[Fernando Lopez-Lezcano <nando@xxxxxxxxxxxxxxxxxx>]

On 02/16/2012 04:33 AM, David Timms wrote:
> On 15/02/12 16:49, Brian Monroe wrote:
> > On Tue, Feb 14, 2012 at 12:21 PM, Brendan Jones
> > <brendan.jones.it@xxxxxxxxx>wrote:
> > > Latency times are really relative things based on many factors. Have you
> > > been getting better latency times than ta
> > yeah.
>
> I would be good to see real numbers associated with claims like the
> above; also how it is measured..

Cyclictest is widely used for measuring latencies:
  https://rt.wiki.kernel.org/articles/c/y/c/Cyclictest.html

> Also, indicate those various kernel patches and or options they use or
> have tested.

The rt patches for the latest kernels (3.2 at this point) are here:
  http://www.kernel.org/pub/linux/kernel/projects/rt/3.2/

This is pretty much all I add to the rt patched kernels for Fedora (with 
the proper configuration options, of course, I'm attaching the extra 
options I'm currently using). To get the best performance in the Jack 
world you need to tune the irq kernel thread priorities, that is usually 
done using rtirq (http://www.rncbc.org/jack/#rtirq), and of course jackd 
should run with the proper rt priority as well.

-- Fernando

# This file is intentionally left empty in the stock kernel. Its a nicety
# added for those wanting to do custom rebuilds with altered config opts.

###
# Planet CCRMA low latency kernel options
###

CONFIG_IRQ_FORCED_THREADING=y

# do not add debugging options
# CONFIG_DEBUG_PREEMPT is not set
# CONFIG_PREEMPT_TRACER is not set

# enable rt preemption
CONFIG_PREEMPT_RT_BASE=y
CONFIG_PREEMPT_RT_FULL=y

# "Testing module to detect hardwareinduced latencies"
CONFIG_HWLAT_DETECTOR=m

# enable histograms for now

# "Interrupts-off Latency Histogram"
CONFIG_INTERRUPT_OFF_HIST=y

# "Preemption-off Latency Histogram"
CONFIG_PREEMPT_OFF_HIST=y

# "Scheduling Latency Histogram"
CONFIG_WAKEUP_LATENCY_HIST=y

# "Missed Timer Offsets Histogram"
CONFIG_MISSED_TIMER_OFFSETS_HIST=y

# taken from old rt configurations for 2.6.33
CONFIG_RCU_BOOST=y
# we also need these (just taking the defaults for now):
CONFIG_RCU_BOOST_PRIO=1
CONFIG_RCU_BOOST_DELAY=500

CONFIG_X86_PAE=y

# CONFIG_DEBUG_SLAB is not set
# CONFIG_RCU_CPU_STALL_VERBOSE is not set

### new options

# from the old 2.6.33 configuration
CONFIG_XEN_DEV_EVTCHN=m
CONFIG_XEN_SYS_HYPERVISOR=y

### temporary options, 2011.08.04, rt7, should be removed on > rt8

# disable two modules to make rt7 build: this is the type of error reported
# ERROR: "of_find_property" [xxx] undefined!
#
# Open Firmware serial port
# CONFIG_SERIAL_OF_PLATFORM is not set
# 
# Multiple Supply Hot Swap Controller with I2C Compatible Monitoring
# CONFIG_SENSORS_LTC4245 is not set

# allow sysrq output to affect latency, we are testing anyway
CONFIG_MAGIC_SYSRQ_FORCE_PRINTK=y

# "m" is an invalid option for 3.0.9 + rt25
# CONFIG_HID_ACRUX_FF is not set

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