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

[Nick Piggin <piggin@cyberone.com.au>]

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 „loaded“ and an „unloaded“ system. The 
>„load“ 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.