Hello, I think I found a performance issue in kernel. This problem was introduced in 3.2.6 and is affecting all version, including 3.7.1 (latest tested).
I measured several kernel builds with a homebrew LAMP platform benchmark (250 runs, average value kept). Kernel 3.2.0 was on production so far, and I tried an upgrade to 3.7.1 (latest kernel at that time). No software other than the kernel was modified. Kernel was built with the same .config file (new options left with default value). Difference in performance was quite huge : Kernel 3.7.1 : ~ 3250 queries / second Kernel 3.2.0 : ~ 4300 queries / second Yes, this is a 25% performance drop ... To narrow things down, I tested several kernels : 3.2.0 to 3.2.5 : OK 3.2.6, 3.2.11, 3.2.28, 3.2.36, 3.7.1 : PERFORMANCE DROP On faulty kernels, performance drop is always the same. At this step, I know that bug was introduced in version 3.2.6. I then used git bisect (amazing tool btw) to find the faulty commit : f51d67a64f32cd81ea8b67ca964fb7cf7e783b2e PM / QoS: CPU C-state breakage with PM Qos change Next test : I used a 3.2.6 and reverted this patch : performance was back to normal. I also reverted this patch on 3.2.36 (latest 3.2.X), and performance was also OK. I was unable to revert this patch on 3.7.1 (structure of the source file /include/linux/pm_qos.h changed too heavily). Kernel was built on a x86_64 platform with binutils 2.19.1 and gcc 4.4.0. .config file used : https://gist.github.com/4567342 Benchmark was run on a dual CPU INTEL L5630 with 4GB of RAM What can I do to resolve this problem ? Thanks ! Olivier -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
