Linux Foundation End User Summit 2008

Real-Time Linux Latency Comparisons

Test System

4 x AMD Opteron(tm) Processor 275 @ 2193.158 MHz
7852808 kB Memory

Kernels and Load

RHEL

2.6.18-53.el5--IDLE
2.6.18-53.el5--LOADED
2.6.24.7-75.el5rt--IDLE
2.6.24.7-75.el5rt--LOADED

SLES

2.6.16.60-0.21-smp--IDLE
2.6.16.60-0.21-smp--LOADED
2.6.22.19-20080722_SLERT10_SP2_rt--IDLE
2.6.22.19-20080722_SLERT10_SP2_rt--LOADED

Tests

The following selection of real-time tests from the LTP test suite were run on the kernels listed above.

gtod_infinite

/*
 * Simple program to measure the time between several pairs of calls to 
 * gettimeofday().  If the average delta is greater than just a few 
 * microseconds on an unloaded system, then something is probably wrong.
 */

SLES	2.6.16.60-0.21-smp--IDLE	2.6.16.60-0.21-smp--LOADED	2.6.22.19-20080722_SLERT10_SP2_rt--IDLE	2.6.22.19-20080722_SLERT10_SP2_rt--LOADED
Scatter Plots
Histograms
Data Files	out.log samples.dat samples.plt	out.log samples.dat samples.plt	out.log samples.dat samples.plt	out.log out.log.bac samples.dat samples.plt

sched_latency

/*
 * Measure the latency involved with periodic scheduling.
 * Steps:
 * - A thread is created at a priority of 89.
 * - It periodically sleeps for a specified duration(PERIOD).
 * - The delay is measured as
 *
 *        delay = (now - start - i*PERIOD) converted to microseconds
 *
 * where, now = CLOCK_MONOTONIC gettime in ns, start = CLOCK_MONOTONIC gettime
 * at the start of the test, i = iteration number, PERIOD = the period chosen
 */

RHEL	2.6.18-53.el5--IDLE	2.6.18-53.el5--LOADED	2.6.24.7-75.el5rt--IDLE	2.6.24.7-75.el5rt--LOADED
Test Log	------------------------------- Scheduling Latency ------------------------------- Running 100000 iterations with a period of 5 ms Periodic load duration: 1 ms Expected running time: 500 s jvmsim disabled Start: 763 us: FAIL Min: 21 us: PASS Max: 2011 us: FAIL Avg: 1017 us: FAIL StdDev: 406.0719 us Quantiles: 99.0% < 1867 99.9% < 1964 99.99% < 1995 99.999% < 2011 Failed Iterations: 99614 Criteria: latencies < 100 us Result: FAIL	------------------------------- Scheduling Latency ------------------------------- Running 100000 iterations with a period of 5 ms Periodic load duration: 1 ms Expected running time: 500 s jvmsim disabled Start: 901 us: FAIL Min: 27 us: PASS Max: 3317 us: FAIL Avg: 1016 us: FAIL StdDev: 400.7819 us Quantiles: 99.0% < 1871 99.9% < 1980 99.99% < 2173 99.999% < 3317 Failed Iterations: 99712 Criteria: latencies < 100 us Result: FAIL	------------------------------- Scheduling Latency ------------------------------- Running 100000 iterations with a period of 5 ms Periodic load duration: 1 ms Expected running time: 500 s jvmsim disabled Start: 9 us: PASS Min: 8 us: PASS Max: 15 us: PASS Avg: 8 us: PASS StdDev: 0.5140 us Quantiles: 99.0% < 9 99.9% < 10 99.99% < 10 99.999% < 15 Failed Iterations: 0 Criteria: latencies < 100 us Result: PASS	------------------------------- Scheduling Latency ------------------------------- Running 100000 iterations with a period of 5 ms Periodic load duration: 1 ms Expected running time: 500 s jvmsim disabled Start: 18 us: PASS Min: 8 us: PASS Max: 41 us: PASS Avg: 17 us: PASS StdDev: 4.9236 us Quantiles: 99.0% < 27 99.9% < 31 99.99% < 37 99.999% < 41 Failed Iterations: 0 Criteria: latencies < 100 us Result: PASS
Scatter Plots
Histograms
Data Files	hist.dat hist.plt out.log samples.dat samples.plt	hist.dat hist.plt out.log samples.dat samples.plt	hist.dat hist.plt out.log samples.dat samples.plt	hist.dat hist.plt out.log samples.dat samples.plt

async_handler

/*
 * Measure the latency involved with asynchronous event handlers. Specifically
 * it measures the latency of the pthread_cond_signal call until the signalled
 * thread is scheduled.
 */

SLES	2.6.16.60-0.21-smp--IDLE	2.6.16.60-0.21-smp--LOADED	2.6.22.19-20080722_SLERT10_SP2_rt--IDLE	2.6.22.19-20080722_SLERT10_SP2_rt--LOADED
Test Log	----------------------------------- Asynchronous Event Handling Latency ----------------------------------- Failed to init mutex: 95 (Operation not supported) jvmsim disabled Running 1000000 iterations Failed to init mutex: 95 (Operation not supported) Failed to init mutex: 95 (Operation not supported) recording statistics... Min: 5 us Max: 20 us handler thread exiting Avg: 5.7381 us StdDev: 0.5929 us signal thread exiting Criteria: latencies < 100 Result: PASS	----------------------------------- Asynchronous Event Handling Latency ----------------------------------- Failed to init mutex: 95 (Operation not supported) jvmsim disabled Running 1000000 iterations Failed to init mutex: 95 (Operation not supported) Failed to init mutex: 95 (Operation not supported) recording statistics... Min: 4 us Max: 8879 us Avg: 12.6853 us StdDev: 9.6024 us handler thread exiting signal thread exiting Criteria: latencies < 100 Result: FAIL	----------------------------------- Asynchronous Event Handling Latency ----------------------------------- jvmsim disabled Running 1000000 iterations recording statistics... Min: 3 us Max: 46 us handler thread exiting Avg: 11.0715 us StdDev: 2.6090 us signal thread exiting Criteria: latencies < 100 Result: PASS	----------------------------------- Asynchronous Event Handling Latency ----------------------------------- jvmsim disabled Running 1000000 iterations recording statistics... Min: 3 us Max: 55 us handler thread exiting Avg: 9.7418 us StdDev: 3.0278 us signal thread exiting Criteria: latencies < 100 Result: PASS
Scatter Plots
Histograms
Data Files	hist.dat hist.plt out.log samples.dat samples.plt	hist.dat hist.plt out.log samples.dat samples.plt	hist.dat hist.plt out.log samples.dat samples.plt	hist.dat hist.plt out.log samples.dat samples.plt