If there is only one cpu and the IOwait is 99%, is the cpu still able to execute other processes, if so, is only 1% cpu resource could be used? or the other process could also use 100% cpu resource although under the scene of 99% Iowait.
I read the following line in one book"
iowait is time spent receiving and handling hardware interrupts as a
percentage of processor ticks.
If so, the 99% iowait doesn't mean the cpu is idle or waiting, actually it's very busy to receive and handle the interrupts. If this is true, I guess the other processes only have 1% cpu resource can be used.
I got the answer: The 99% IOWait means cpu almost 99% idle. i.e IOWait is a subset of cpu Idle. The following url has an excellent explanation:
http://veithen.github.io/2013/11/18/iowait-linux.html
Related
I'm using htop to monitor the CPU usage of my task. However, the CPU% value exceed 100% sometimes, which really confused me.
Some blogs explain that this is because I'm using a multi-core machine(this is true). If there are 8 (logic) cores, the max value of CPU% is gonna be 800%. CPU% over 100% means that my task is occupying more than one core.
But my question is: there is a column named CPU in htop window which shows the id of the core my task is running on. So how can the usage of this single core exceed 100%.
This is the screenshot. You can see the 84th core's usage is 375%!
I am trying to measure the impact of CPU scheduler on a large AI program (https://github.com/mozilla/DeepSpeech).
By using strace, I can see that it uses a lot of (~200) CPU threads.
I have tried using Linux Perf to measure this, but I have only been able to find the number of context switch events, not the overhead of them.
What I am trying to achieve is the total CPU core-seconds spent on context switching. Since it is a pretty large program, I would prefer non-invasive tools to avoid having to edit the source code of this program.
How can I do this?
Are you sure most of those 200 threads are actually waiting to run at the same time, not waiting for data from a system call? I guess you can tell from perf stat that context-switches are actually pretty high, but part of the question is whether they're high for the threads doing the critical work.
The cost of a context-switch is reflected in cache misses once a thread is running again. (And stopping OoO exec from finding as much ILP right at the interrupt boundary). This cost is more significant than the cost of the kernel code that saves/restores registers. So even if there was a way to measure how much time the CPUs spent in kernel context-switch code (possible with perf record sampling profiler as long as your perf_event_paranoid setting allows recording kernel addresses), that wouldn't be an accurate reflection of the true cost.
Even making a system call has a similar (but lower and more frequent) performance cost from serializing OoO exec, as well as disturbing caches (and TLB). There's a useful characterization of this on real modern CPUs (from 2010) in a paper by Livio & Stumm, especially the graph on the first page of IPC (instructions per cycle) dropping after a system call returns, and taking time to recover: FlexSC: Flexible System Call Scheduling with Exception-Less System Calls. (Conference presentation: https://www.usenix.org/conference/osdi10/flexsc-flexible-system-call-scheduling-exception-less-system-calls)
You might estimate context-switch cost by running the program on a system with enough cores not to need to context-switch much at all (e.g. a big many-core Xeon or Epyc), vs. on fewer cores but with the same CPUs / caches / inter-core latency and so on. So, on the same system with taskset --cpu-list 0-8 ./program to limit how many cores it can use.
Look at the total user-space CPU-seconds used: the amount higher is the extra amount of CPU time needed because of slowdowns from context switched. The wall-clock time will of course be higher when the same work has to compete for fewer cores, but perf stat includes a "task-clock" output which tells you a total time in CPU-milliseconds that threads of your process spent on CPUs. That would be constant for the same amount of work, with perfect scaling to more threads, and/or to the same threads competing for more / fewer cores.
But that would tell you about context-switch overhead on that big system with big caches and higher latency between cores than on a small desktop.
I'm really confused. Why does the load average and %CPU does not match the process CPU usage below. It seems like the process is eating up a lot of CPU while the AWS EC2 meters only says 25% CPU is used.
%CPU -- CPU Usage : The percentage of your CPU that is being used by the process. By default, top displays this as a percentage
of a single CPU. On multi-core systems, you can have percentages
that are greater than 100%. For example, if 3 cores are at 60% use,
top will show a CPU use of 180%.
You can toggle this behavior by hitting Shift+i while top is running to show the overall percentage of available
CPUs in use.
load average: 22.56, 24.99, 26.51
From left to right, these numbers show you the average load over the last 1 minute, the last 5 minutes, and the last 15 minutes.
us -- User CPU time
The time the CPU has spent running users' processes that are not niced.
sy -- System CPU time
The time the CPU has spent running the kernel and its processes.
ni -- Nice CPU time
The time the CPU has spent running users' proccess that have been niced.
wa -- iowait
Amount of time the CPU has been waiting for I/O to complete.
hi -- Hardware IRQ
The amount of time the CPU has been servicing hardware interrupts.
si -- Software Interrupts
The amount of time the CPU has been servicing software interrupts.
st -- Steal Time
The amount of CPU 'stolen' from this virtual machine by the hypervisor for other tasks (such as running another virtual machine).
See more details from In Linux “top” command what are us, sy, ni, id, wa, hi, si and st (for CPU usage).
after you run command "top" you can press "1" on your keyboard to see individual CPU utilization, more details when you run command "man top"
Note process "msqld" can use CPU from several resources and its utilization % could easily go beyond 100% in "top" display.
Hi maybe your app using single core and other cores are free. I think your instance has 4 CPU core and one is utilizing 100%. can you please check utilization by each core.
I have a quad core (with hyper threading Technology - HT)
I'm running an application which takes 270% CPU (according to TOP command)
What is the total available CPU usage? (is it 400% or 800%?)
I'm asking because according to Intel documentation, the HT can up the performance up to 30% cpu, so 800% seem to much, yeah?)
What is the relation between load averages and CPU usage?
1: 800. You have 8 cores visible to the OS - that they are not real cores (due to hyperthreading limitations) is not of concern.
2: Ever bothered reading documentation? Practically there is no relation between load average and CPU uage. Load average is "waiting processes" but that can mean they are waiting for IO, and the CPU may not be busy.
Here is a picture of CPU monitoring provided by VisualVm profiler. I'm confused because I can't understand what does percentage means?As you can see for CPU the number is 24,1 of what?overall cpu time?and gc - 21,8 the same question.What is 100% in both cases?Please clarify this data.
CPU usage is the total CPU usage of your system. The GC activity shows how much of the CPU is spent by the GC threads.
In your example, it looks like the GC was executing and thus contributing to the majority of the total CPU usage in the system. After the GC finished, there was no CPU used.
You should check if this observation is consistent with your GC logs, the logs will shows you the activity around the 19:50 time.