Messages in this thread | | | Subject | Re: CONFIG_PREEMPT and server workloads | From | Tom Sightler <> | Date | Thu, 18 Mar 2004 18:54:30 -0500 |
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On Thu, 2004-03-18 at 10:37, Andrea Arcangeli wrote: > On Thu, Mar 18, 2004 at 10:20:23AM -0500, Tom Sightler wrote: > > Well, I reported an issue on my laptop several weeks ago where network > > activity via my aironet wireless adapter would use 60-70% of the CPU but > > only when PREEMPT was enabled. Looking back over the list I see other > > sounds like a preempt bug triggering in a softirq, or maybe an SMP bug > triggering in UP. > > I certainly agree with Andrew that preempt cannot cause a 60/70% > slowdown with a network card at least (if you do nothing but spinlocking > in a loop then it's possible a 60/70% slowdown instead but the system > time that a wifi card should take is nothing compared to the idle/user > time).
Actually, I managed to get two bugs mixed together, my system had a problem with PREEMPT and ACPI but this certainly seems to be fixed in the current kernels, at least in 2.6.3-mm2 (my current everyday kernel) and 2.6.5-rc1-mm2 (just a quick test tonight). Somehow the combination of PREEMPT and any little applet that used ACPI (battery monitor) would use a lot of CPU, then using the aironet adapter caused the CPU to spike. Turning off either PREEMPT or ACPI would get rid of the problem. I forgot about the ACPI part being in the mix.
I still think the aironet driver acts strange, but I'm not really sure how to describe it. In 2.6.3-mm2 which I transfer a file top will show 60% of the cpu time in 'irq' whatever that means. It seems vmstat still calls this idle. Perhaps I'm interpreting those numbers wrong.
When I enable preempt on the same system my network performance with the aironet driver drops tremendously, probably by 75%, and the rest of the systems seems to crawl. Somehow without preempt this isn't a problem, even though it shows 60% of the time in 'irq' the rest of the system feels responsive. What would be the proper way to track down this type of problem?
I'm having more problems with 2.6.5-rc1-mm2 and the aironet card. It seems to preform poorly no matter if preempt is on or off. I'm still looking into that. I don't think the drivers has changed significantly between the two version so I'm hoping I just made a mistake.
Later, Tom
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