On Nov 24, 2016 2:18 AM, "Greg KH" <greg@kroah.com> wrote:
On Thu, Nov 24, 2016 at 02:01:41AM -0500, Kenneth Adam Miller wrote:
Hello,
I have a scheduler issue in two different respects:
1) I have a process that is supposed to tight loop, and it is being given very very little time on the system. I don't want that - I want those who would use the processor to be given the resources to run as fast as they each can.
What is causing it to give up its timeslice? Is it waiting for I/O? Doing something else to sleep?
It's multithreaded, so it reads in a loop in one thread and writes in another thread. What I saw when I ran strace on it is each process would run for too long- the program is designed to try and stay out of the kernel on each side, so it checks some shared variables before it ever goes.
2) I am seeing with perf that the maximum overhead at each section does not sum up to be more than 15 percent. Total, probably something like 18% of cpu time is used, and my binary has rocketed in slowness from about 2 seconds or less total to several minutes.
What changed to make things slower? Did you change kernel versions or did you change something in your userspace program?
The kernel versions specifically couldnt have anything to do with it but it was different kernels. The test runs in less that 2 seconds on my host. When I copy it to our custom linux, it takes minutes for it to run. I think it's some extra setting that we're missing while building the kernel, and I don't know what that is. I got a huge improvement when I changed the multicore scheduling to allow preemption "(desktop)" but there's still a problem as I've described with one of the processes not using the core as it should.
I think that the linux scheduler isn't scheduling it, because this process is just some unit tests that double as benchmarks in that they shm_open a file and write into it with memcpy's.
Are you sure that I/O isn't happening here like through swap or something else?
Well, we're using tmpfs and don't have a disk in the machine, but I will say this process is using all lot of the address space. One problem here is that the kernel has more ram than it thinks it does, but what I want to emphasize is that I haven't changed the program to allocate any more than it was previously. I'm not sure if that's a kernel change or some setting, but it went from 85% to 98%. The reason why is that there is a large latency even without that big program in there; I can't run my standalone tests in qemu without it also taking minutes. I understand qemu has to emulate, and that's its not just a VM, but I'm going from host CPU to guest, and the settings are the same.
What does perf say is taking all of your time?
When I ran perf what it appeared to indicate is that the largest consumer of time was my library, which should be right in either scenario because it should use stay out of the kernel as I've designed it. In addition, the work takes place there anyway, so that's right. What's not right is the fact that the largest percent of time used is around 15%, and all the others combined don't add up to anything near 100.
thanks,
greg k-h