On 31 March 2015 at 22:53, Dave Johansen <davejohansen@xxxxxxxxx> wrote:
On Tue, Mar 31, 2015 at 3:26 PM, Richard W.M. Jones <rjones@xxxxxxxxxx> wrote:On Tue, Mar 31, 2015 at 12:21:55PM -0700, Dave Johansen wrote:
> You're right that is a problem because my "purely CPU bound task" was
> actually writing to disk every 10 seconds, so I've attached an updated
> version that pre-allocates a vector and stores the results there so they
> can be dumped when the users presses Ctrl-C. With this update, the "CPU
> bound task" should only using CPU and existing memory but I still see the
> same slow down in the "CPU bound task" when the disk I/O is happening.
For the definitive test, you might want to add a call to mlockall()
into your program. It is supposed to lock every page of your process
into RAM (just in case it is being swapped out, and hence using I/O).
I guess you will also need to run the CPU test program as root.I added the call to mlockall() (it did have to be run as root) on a F21 machine with no swap and the slow down was still visible in the "CPU bound task".
The slowdown isn't going to go away with mlockall etc. All that is to do is so you can have a better idea of where the slowdown is by removing various noise. There isn't going to be any quick fix for the slowdown. At best you can figure out the points in the kernel that might need fixing.. but realize this is a problem which has been around for at least a decade and a half. It has been looked at in various forms over and over and over again by kernel devels. It isn't going to be fixed quickly or easily so expect that if you are interested in figuring it out and helping the devels that there isn't going to be an quick fix. It is going to be a long hard slog.
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Stephen J Smoogen.
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