| The most reliable way of running benchmarks is to do it in an otherwise idle |
| system. On a busy system, the results will vary according to the other tasks |
| demanding attention in the system. |
| |
| We have managed to obtain quite reliable results by doing the following on |
| Linux (and you need root): |
| |
| - switching the scheduler to a Real-Time mode |
| - setting the processor affinity to one single processor |
| - disabling the other thread of the same core |
| |
| This should work rather well for CPU-intensive tasks. A task that is in Real- |
| Time mode will simply not be preempted by the OS. But if you make OS syscalls, |
| especially I/O ones, your task will be de-scheduled. Note that this includes |
| page faults, so if you can, make sure your benchmark's warmup code paths touch |
| most of the data. |
| |
| To do this you need a tool called schedtool (package schedtool), from |
| http://freequaos.host.sk/schedtool/ |
| |
| From this point on, we are using CPU0 for all tasks: |
| |
| If you have a Hyperthreaded multi-core processor (Core-i5 and Core-i7), you |
| have to disable the other thread of the same core as CPU0. To discover which |
| one it is: |
| |
| $ cat /sys/devices/system/cpu/cpu0/topology/thread_siblings_list |
| |
| This will print something like 0,4, meaning that CPUs 0 and 4 are sibling |
| threads on the same core. So we'll turn CPU 4 off: |
| |
| (as root) |
| # echo 0 > /sys/devices/system/cpu/cpu4/online |
| |
| To turn it back on, echo 1 into the same file. |
| |
| To run a task on CPU 0 exclusively, using FIFO RT priority 10, you run the |
| following: |
| |
| (as root) |
| # schedtool -F -p 10 -a 1 -e ./taskname |
| |
| For example: |
| # schedtool -F -p 10 -a 1 -e ./tst_bench_qstring -tickcounter |
| |
| Warning: if your task livelocks or takes far too long to complete, your system |
| may be unusable for a long time, especially if you don't have other cores to |
| run stuff on. To prevent that, run it before schedtool and time it. |
| |
| You can also limit the CPU time that the task is allowed to take. Run in the |
| same shell as you'll run schedtool: |
| |
| $ ulimit -s 300 |
| To limit to 300 seconds (5 minutes) |
| |
| If your task runs away, it will get a SIGXCPU after consuming 5 minutes of CPU |
| time (5 minutes running at 100%). |
| |
| If your app is multithreaded, you may want to give it more CPUs, like CPU0 and |
| CPU1 with -a 3 (it's a bitmask). |
| |
| For best results, you should disable ALL other cores and threads of the same |
| processor. The new Core-i7 have one processor with 4 cores, |
| each core can run 2 threads; the older Mac Pros have two processors with 4 |
| cores each. So on those Mac Pros, you'd disable cores 1, 2 and 3, while on the |
| Core-i7, you'll need to disable all other CPUs. |
| |
| However, disabling just the sibling thread seems to produce very reliable |
| results for me already, with variance often below 0.5% (even though there are |
| some measurable spikes). |
| |
| Other things to try: |
| |
| Running the benchmark with highest priority, i.e. "sudo nice -19" |
| usually produces stable results on some machines. If the benchmark also |
| involves displaying something on the screen (on X11), running it with |
| "-sync" is a must. Though, in that case the "real" cost is not correct, |
| but it is useful to discover regressions. |
| |
| Also; not many people know about ionice (1) |
| ionice - get/set program io scheduling class and priority |