# Ngx\_cpuinfo / kvm & qemu / ngx\_cacheline\_size **URL:** \ **Category:** NGINX\ **Created:** [December 2, 2012, 12:15pm UTC](https://www.ruby-forum.com/t/ngx-cpuinfo-kvm-qemu-ngx-cacheline-size/224918 "2012-12-02T12:15:50Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\ **Post date:** [December 2, 2012, 12:15pm UTC](https://www.ruby-forum.com/t/ngx-cpuinfo-kvm-qemu-ngx-cacheline-size/224918/1 "2012-12-02T12:15:50Z")
Hi, I have virtual machine running on qemu / kvm. cat /proc/cpuinfo give : processor : 0 vendor\_id : GenuineIntel cpu family : 6 model : 2 model name : QEMU Virtual CPU version 1.1.2 stepping : 3 microcode : 0x1 cpu MHz : 1999.999 cache size : 4096 KB fpu : yes fpu\_exception : yes cpuid level : 4 wp : yes flags : fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pse36 clflush mmx fxsr sse sse2 syscall nx lm up rep\_good nopl pni cx16 popcnt hypervisor lahf\_lm bogomips : 3999.99 clflush size : 64 cache\_alignment : 64 address sizes : 40 bits physical, 48 bits virtual power management: the code in ngx\_cpuinfo use the model value (2), and set the ngx\_cacheline\_size to 32, instead of 64 given by cache\_alignment in /proc/cpuinfo. On same host using vmware virtualization, /proc/cpuinfo give a model of 45, ngx\_cacheline\_size is set to 64. I think I can have low performances problems due to the ngx\_cacheline\_size of 32 under qemu virtualization. Do you confirm ? Do you think we have to change ngx\_cpuinfo code to give ngx\_cacheline\_size 64 on qemu env ? Regards, Bertrand ---
**Author:** ![Bertrand\_P](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e19b73/32.png) [@Bertrand\_P](https://www.ruby-forum.com/u/Bertrand_P)\ **Post date:** [December 3, 2012, 2:55pm UTC](https://www.ruby-forum.com/t/ngx-cpuinfo-kvm-qemu-ngx-cacheline-size/224918/2 "2012-12-03T14:55:56Z")
Hello! On Sun, Dec 02, 2012 at 12:15:03PM +0100, Bertrand P. wrote: > stepping : 3 > bogomips : 3999.99 > ngx\_cacheline\_size is set to 64. > > I think I can have low performances problems due to the ngx\_cacheline\_size > of 32 under qemu virtualization. Do you confirm ? > > Do you think we have to change ngx\_cpuinfo code to give ngx\_cacheline\_size > 64 on qemu env ? Incorrectly detected cache line size may result in slightly non-optimal performance in some situations (I wouldn’t suppose 32 vs 64 to result in more than a few percent difference in real word nginx operation though, even on real hardware). On the other hand, using virtualization ensures non-optimal performance in all situations, so one using virtualization probably don’t care anyway. (In any case there are no plans to change Intel CPUs cache line size detection code to support fake emulated qemu CPUs. In particular because it’s not possible to tell which cache line size is more effective for a given emulated CPU. It’s up to you to configure qemu to emulate a CPU with a cache line size most effective on your hardware if you think you care.) – Maxim D. > **[F5 NGINX: Built for cloud-native apps. Ready for AI traffic.](https://www.f5.com/products/nginx)** > > Deliver modern applications with NGINX. Explore high-performance load balancing, API gateways, and web server solutions trusted by top global enterprises. ---
**Author:** ![Bertrand\_P](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e19b73/32.png) [@Bertrand\_P](https://www.ruby-forum.com/u/Bertrand_P)\ **Post date:** [December 3, 2012, 4:06pm UTC](https://www.ruby-forum.com/t/ngx-cpuinfo-kvm-qemu-ngx-cacheline-size/224918/3 "2012-12-03T16:06:39Z")
Ok, thx you for answering. Regards, Bertrand