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SubjectRe: (disk/cpu) kernel performance problem


On Thu, 29 Jul 1999, Stephen C. Tweedie wrote:

> Hi,
>
> On Wed, 28 Jul 1999 16:12:59 -0700 (PDT), Gerald Aigner
> <aigner@stanford.edu> said:
>
> > I just profiled the 2.3.11 kernel and around 37% of the time was spend in
> > file_read_actor. By looking at the function I discovered that this
> > function is responsible for copying data from the kernel to the user
> > space.
>
> > Here the basic performance of this function:
> > data transfered: 1.8 GB
> > total execution time of the function: 23 seconds
> > ---------------------------------------------------
> > throughput per second: 78 MB/sec
>
> > This means that the Linux memory copy function used for copying
> > disk data achieves a memory bandwidth of approximately 156 MB/sec
> > on a system with a 66 MHz system bus.
>
> Remember that you are doing disk IO here too so there is also 78MB/sec
> of bus traffic going between the scsi controller and main memory.
> That's also going to reduce the available bandwidth. Try doing a copy
> purely out of cache to see what the real memory bandwidth available via
> the filesystem is.
>
The total execution time of a cached loading is 20 seconds.

This brings the throughput to 90 MB/second and therefore the
memory bandwidth to 180 MB/second (the kernel is 2.3.11, and the
reading is done in 10 MB chunks, in two threads).

-Gerald



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