Bug 51691
| Summary: | High cpu overhead on using multiple cores | ||
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| Product: | Process Management | Reporter: | sworddragon2 |
| Component: | Scheduler | Assignee: | Ingo Molnar (mingo) |
| Status: | REOPENED --- | ||
| Severity: | normal | CC: | alan, gmohankumarg, pruzinat, szg00000 |
| Priority: | P1 | ||
| Hardware: | All | ||
| OS: | Linux | ||
| URL: | https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1073148 | ||
| Kernel Version: | 3.7 | Subsystem: | |
| Regression: | No | Bisected commit-id: | |
| Attachments: |
Example script
Log of top using 1 and 2 cores Example in C Example in C Testcase |
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Created attachment 89111 [details]
Log of top using 1 and 2 cores
You need to profile your application and interpreter at a low level - most likely you've got contention problems. Doesn't seem to be a kernel bug here I have now port the script to C to exclude Python as the source of the problem. The bug appears there too. My cpu usage on 1 core with 2 threads is ~14% and on 2 cores with 2 threads ~21%. Created attachment 90241 [details]
Example in C
Please close the bug, not a kernel problem. Infinite loop of usleeps(0.0000001) will in fact cause 'slight overhead'. usleep defined ranges [0,1000 000], not sure how did that float went unnoticed by compiler. nevermind my last comment. Don't worry - you have helped me to find a potential error in my testcase. I have updated it to use nanosleep() instead but the result is still the same - multiple cores will produce much overhead. Created attachment 99371 [details]
Example in C
Here is an updated version of the testcase. It uses now number of online core * 2 as threads because the overhead gets much higher with more threads. Here are the results on my system with 12 threads: Calling ./test will result in a cpu usage of ~185%/600%. Calling taskset -c 0 ./threads will result in a cpu usage of ~30%/600%. Created attachment 115241 [details]
Testcase
Compiling with "gcc -o threads ./threads.c -lpthread".
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Created attachment 89101 [details] Example script On my AMD Phenom II X6 1045T if a process uses multiple cores it has a very high cpu overhead (an average of 50%) compared to limiting the process to 1 cpu core. In the attachments is: - The script which was used for the test. It creates 2 threads which are in semi-idle (using ~10-20% of one core per thread on my system). - A log which shows top with a process (the script) which needed more cpu power before it was limited with taskset to 1 core.