btorpey / clocks

Code to test various clocks

Code to test various clocks under Linux (and OSX)

Some quick notes on the code:

• The ClockTest.sh script compiles the code, and executes the test programs. It uses taskset to ensure that the tests don't migrate to different CPUs. (taskset is done only for the C++ test – the JVM uses so many threads that pinning them all to a single CPU is probably not a good idea). You'll need to modify the script to specify the correct location for JAVA_HOME on your system, or set JAVA_HOME prior to calling the script.

• The script detects whether the rdtscp instruction is available by querying /proc/cpuinfo. If not, it bypasses that code (which would otherwise throw a SIGILL).

• You can also pass in the CPU frequency, in GHz. If you don't, then a frequency of 1 is used, which has the effect of returning the tick count unmodified for those clock sources that use tick counts (i.e., anything having to do with the RDTSC instruction).

• If the script doesn't seem to be working, execute it with “bash -xv ClockTest.sh” to see the actual commands.

© Copyright 2014 by Bill Torpey. All Rights Reserved.

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

clocks's Issues

Fix for crash if BUCKETS changed

The code does a bit-wise AND with 255 of the iteration step to find the location where to store the sample; this obviously only works if the number of buckets is 255, and usually leads to crazy results or crashes otherwise.
The fix is trivial - do a modulo operation instead. Code diff below:

diff --git a/ClockBench.cpp b/ClockBench.cpp
index 8b73921..680e641 100755
--- a/ClockBench.cpp
+++ b/ClockBench.cpp
@@ -168,7 +168,7 @@ int main(int argc, char** argv)

    {
       for (int i = 0; i < ITERS * BUCKETS; ++i) {
-         int n = i & 0xff;
+         int n = i % BUCKETS;
          timestamp[n] = cpuid_rdtsc();
       }
       for (int i = 0; i < BUCKETS; ++i) {
@@ -183,7 +183,7 @@ int main(int argc, char** argv)
    #ifdef RDTSCP
    {
       for (int i = 0; i < ITERS * BUCKETS; ++i) {
-         int n = i & 0xff;
+         int n = i % BUCKETS;
          timestamp[n] = rdtscp();
       }
       for (int i = 0; i < BUCKETS; ++i) {
@@ -193,11 +193,11 @@ int main(int argc, char** argv)
       x.print();
       x.dump(file);
    }
-   #endif
+  #endif

    {
       for (int i = 0; i < ITERS * BUCKETS; ++i) {
-         int n = i & 0xff;
+         int n = i % BUCKETS;
          timestamp[n] = rdtsc();
       }
       for (int i = 0; i < BUCKETS; ++i) {