Re: 2.6.1 Scheduler Latency Measurements (Preemption diabled/enabled)

[Nick Piggin]

Christoph Stueckjuergen wrote:

> Hi,
>
> I performed a series of measurements comparing scheduler latency of a 2.6.1 
> kernel with preemption enabled and disabled on an AMD Elan (i486 compatible) 
> with 133 Mhz clock frequency.
>
> The measurements were performed with a kernel module and a user mode process 
> that communicate via a character device interface. The user mode process uses 
> a blocking read() call to obtain data from the kernel. The kernel module 
> reads the system time every 10 ms by calling do_gettimeofday(), wakes up the 
> sleeping user mode process and passes the system time to it. After having 
> received the system time from the kernel, the user mode process reads the 
> system time by calling gettimeofday() and is thus able to determine the 
> scheduler latency by subtracting the two times. The user mode process is run 
> with the SCHED_FIFO scheduling policy.
>
> Measurements were carried out on a âloadedâ and an âunloadedâ system. The 
> âloadâ was created by a process that continuously writes data to the serial 
> interface /dev/ttyS0.
>
> The results are:
> "loaded" system, 10.000 samples
> average scheduler latency (preemption enabled / disabled): 170 us / 232 us
> minimum scheduler latency (preemption enabled / disabled): 49 us / 43 us
> maximum scheduler latency (preemption enabled / disabled): 840 us / 1063 us
>
> "unloaded" system, 10.000 samples
> average scheduler latency (preemption enabled / disabled): 50 us / 44 us
> minimum scheduler latency (preemption enabled / disabled): 46 us / 41 us
> maximum scheduler latency (preemption enabled / disabled): 233 us / 215 us
>
> Any help in interpreting the data would be highly appreciated. Especially:
> - Why does preemption lead to a higher minimum scheduler latency in the loaded 
> case?
> - Why does preemption worsen scheduler latency on the unloaded system?

Because it adds a small amount of overhead. What you are paying for
is the improvement in worst case latencies. Looks like it is exactly
what you would expect.

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