Hi Sam,
Dean and I are looking at trying to push the efficiency of requests
from the kernel module up through the device to client-core. I added
the --threaded option to the client to allow the client-core to run
with multiple threads (one each for bmi, dev, and main -- and also a
remount thread, but lets ignore that for now), so the device thread
should be able to keep pulling requests of the device without having
to wait for bmi operations to complete.
Cool!
This could address some of the performance problems that Phil also had
pointed a while back where multiple outstanding requests were slower
than a single outstanding request.
PINT_dev_test_unexpected takes an incount of 5, so its only going to
read at most 5 requests off the device for each call. Once it
returns, each of the unexpected requests is added to the completed
jobs array and then we signal the jobs completed condition variable
_for each request_. It seems like this will be 5x the number of
context switches between the device thread and the main thread that we
need.
Also, we poll every time before reading another request off the
device. What about trying to read a number of requests off the device
at once with one read (or possibly a readv so we can keep separate
buffers per request).
Hmm.. both of these are good points. I had dabbled with doing a readv a
while back. It might make a difference although I suspect this might be
in the noise region since
if there are requests to be serviced, poll() will only take the time of
a syscall which should be pretty fast these days.. but worth a shot.
Also, it looks like we do a malloc for each new request buffer, and
then a free once we're done with it, and a memset of the info struct.
It seems like we could manage the buffers on the stack instead of the
heap, and save on a few system calls there.
Now we are definitely in the noise region.. :) just kidding. glibc's
malloc implementation should typically amortize overheads in invoking
system calls (sbrk etc).
For both threaded and nonthreaded, with the workload that Dean is
using, he found that the PINT_dev_test_unexpected always returned 5
requests in the outcount. So it looks like there are always requests
sitting on the device, waiting to be read by client-core. Are we just
not able to process requests fast enough through BMI and the state
machines, or is the cost of polling and signaling every time we read a
request off the device slowing us down? In other words, does it make
sense to rework the code a little bit or will we just get bottlenecked
elsewhere?
It is definitely interesting to try all this out, but I am not sure if
the bottlenecks are here or elsewhere.
What does this workload do by the way?
thanks,
Murali
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