>Your system looks pretty much interrupt driven, and I would assume that
>all these are of same importance. So I would actually recommend the
>vanilla kernel, with HZ=100 and preemption turned off.
Hi Steve,
I thought I would update you (and those on the list who care) about my real
world experience comparing the 2.6.14 kernel compiled with the timer
frequency set to 100 versus 1000.
If you recall, I'm running a video server that is taking in and sending out
a large number of streams of video and audio clips that range in data rates
from 3.8 to 28 MB/sec.
We have a benchmark test that we developed that simulates multiple
non-linear video editing workstations playing back sequences of video each from their
own timeline. We developed this test mostly to tune our storage subsystem, so
it's not a perfect test to evaluate timer frequencies (because it doesn't
involve any network traffic).
Each test is really a sequence of scripts that a) run dd on a trio of one
video and two audio files and b) move on to the next script until all scripts
have been executed, each time reading data from the RAIDS and writing the data
to /dev/null until 21 clips worth of data have been read. We do this in
parallel 5, 6, 10, 15 times to simulate that same number of workstations calling
for data simultaneously.
The test is an approximate simulation of video editing workstations playing
back unique sequences of 21 clips that each range in length from 3 to 5
seconds and that run at a rate of 18 MB/sec video (which is roughly 8-bit
uncompressed standard definition).
The test puts extreme stress on our RAID subsystem -- forcing drives to seek
all over the place to play back these random clips (think of each sequence
as a little 90 second story with 21 separate shots).
After conducting many tests over the weekend, we concluded that the tests
consistently took 6 percent longer to complete when running under a kernel
compiled with the 100 hz timer versus an identical kernel compiled with a 1000 hz
timer.
Again, I realize this isn't a perfect simulation of what our server does in
real life; we weren't moving data over a network -- thus there were no NIC
card interrupts involved, no samba or netatalk work taking place, and instead
all of our scripts and dd commands are running on the server.
Still, the results are interesting. That plus the fact that our "user
management applications" open and run excruciatingly slowly when the timer
frequency is 100 versus 1000 (3 x longer to open the application) is making us stick
with a timer frequency of 1000 for now.
Thanks again for your help. I'm sorry if I offended anyone on this list by
writing from an AOL address.
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