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Showing posts from March, 2010

Cisco SLM2024 and Firefox

I don't know what got me to search for new firmware for my Linksys (now Cisco) SLM2024. But I did, and found version (upgrade from the previous 1.0.1). Aside from the web interface, there's no obvious changes. It is now branded Cisco, rather than Linksys. But the most important change for me wasn't in the release notes: I can now manage my gigabit switch using Firefox ! (tested with Firefox 3.6.2 on Windows)

HZ divider effect on timer interrupt overhead

Red Hat and related distro's (like CentOS) use 1000 timer interrupts per second, per CPU core or thread (this is called the "HZ value" inside the kernel). Because this causes a lot of extra work in case of virtualization, and caused (past tense since RHEL 5.4!) problems with timekeeping, the "divider" kernel parameter has been introduced. For example, by booting with "divider=10", the kernel uses 100 timer interrupts instead of 1000, and "divider=25" means 40 timer interrupts per second.
I did a little test today to see what difference that makes when running CentOS5.4 on vSphere. Tests were done with the current 2.6.18-164.11.1.el5 x86_64 kernel in a single vCPU VM. These are the results from my test environment:
standard settings: HZ=1000: 60 MHz cycles used when idle.divider=2: HZ=500: 36 MHz cycles used when idle.
divider=5: HZ=200: 25 MHz cycles used when idle.
divider=10: HZ=100: 20 MHz cycles used when idle.divider=25: HZ=40: around 10-15…