I have two machines, one x86 machine with HZ value 1000, other embedded machine with HZ value 250.
If i use kernel timers on both machines, with
timer.expires = msecs_to_jiffies(x),
So now time expiration time will be same 'x' milli seconds on both machines or do we need special care in these cases.
If you use high resolution timers (kernel configuration), then the kernel timers use it and you'll get a correct result.
If you are not using HR timers, then on HZ=250 system the minimum time slice is 4 ms. So if you sleep for less you'll wake up on the next timeslice. For example if you run a loop 1000 times sleeping for 1ms, the loop will end after 4 sec.
I can take a guess based on the names, but what specifically are wall-clock-time, user-cpu-time, and system-cpu-time in Unix?
Is user-cpu time the amount of time spent executing user-code while kernel-cpu time the amount of time spent in the kernel due to the need of privileged operations (like I/O to disk)?
What unit of time is this measurement in?
And is wall-clock time really the number of seconds the process has spent on the CPU or is the name just misleading?
Wall-clock time is the time that a clock on the wall (or a stopwatch in hand) would measure as having elapsed between the start of the process and 'now'.
The user-cpu time and system-cpu time are pretty much as you said - the amount of time spent in user code and the amount of time spent in kernel code.
The units are seconds (and subseconds, which might be microseconds or nanoseconds).
The wall-clock time is not the number of seconds that the process has spent on the CPU; it is the elapsed time, including time spent waiting for its turn on the CPU (while other processes get to run).
Wall clock time: time elapsed according to the computer's internal clock, which should match time in the outside world. This has nothing to do with CPU usage; it's given for reference.
User CPU time and system time: exactly what you think. System calls, which include I/O calls such as read, write, etc. are executed by jumping into kernel code and executing that.
If wall clock time < CPU time, then you're executing a program in parallel. If wall clock time > CPU time, you're waiting for disk, network or other devices.
All are measured in seconds, per the SI.
time [WHAT-EVER-COMMAND]
real 7m2.444s
user 76m14.607s
sys 2m29.432s
$ lscpu
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 24
real or wall-clock
real 7m2.444s
On a system with a 24 core-processor, this cmd/process took more than 7 minutes to complete. That by utilizing the most possible parallelism with all given cores.
user
user 76m14.607s
The cmd/process has utilized this much amount of CPU time.
In other words, on machine with single core CPU, the real and user will be nearly equal, so the same command will take approximately 76 minutes to complete.
sys
sys 2m29.432s
This is the time taken by the kernel to execute all the basic/system level operations to run this cmd, including context switching, resource allocation, etc.
Note: The example assumes that your command utilizes parallelism/threads.
Detailed man page: https://linux.die.net/man/1/time
Wall clock time is exactly what it says, the time elapsed as measured by the clock on your wall (or wristwatch)
User CPU time is the time spent in "user land", that is time spent on non-kernel processes.
System CPU time is time spent in the kernel, usually time spent servicing system calls.
Normally when the linux system boots up it actually takes the reference time from RTC and runs a software timer on its own [i.e, generally known as system clock/wall clock]. When the system is about to shutdown it sync its wall clock time with RTC. I am looking for a method to implement a wall clock in c as similar to this. Can any body suggest some idea for me?
Thanks in advance,
Anandhakrishnan Ramasamy.
What OS usually do is they fetch the system startup time from RTC or HPET or any other timer device. And after they load PIC or APIC with a value to receive periodic interrupts from them (e.g after every 100ms). Based on these interrupts value of system clock or wall clock gets updated.
You can't do it in plain C without relying on functionalities provided by the OS. The reason is that the OS schedules several applications through multiprogramming, and your C application can't have knowledge about when it has been suspended by the scheduler.
Therefore, you have to use Posix functions like gettimeofday(), time() and so on.
Its hard to do this 100% correctly. You will have to detect times when the CPU goes to sleep, if the system is suspended, and also any time someone changes the timezone, or when daylight savings time starts or ends. You would have to do all these things yourself.
All CPUs today have a high resolution timer. Its just a register that increments every CPU clock cycle. If you know the frequency of the CPU, and you read that register on a regular basis ( e.g. often enough that it doesn't overflow ) you can measure time.
On linux there is a family of functions that reads this register for you, and figures out the CPU frequency, and returns the time in that register in nano-seconds:
timespec ts;
clock_gettime( CLOCK_MONOTONIC_RAW, &ts );
u64 timeInNanoSeconds = ts.tv_nsec + ( ts.tv_sec * 1000000000LL );
That time will wrap around every 5 minutes or so. So you have to read it pretty often, so you can detect the wrap around. So any time you read it, if ts.tv.nsec is smaller than the last time you called it, they you had an overflow, and you have to account for it.
Once you can accurately measure the passage of a second, then you can build your wall clock from there.
i am using msecs_to_jiffies(msecs) to get delay. I need a delay of 16 ms. But the problem is the function return 1 for input 1-10, 2 for 11-20, 3 for 21-30 and son on. Hence i am unable to set proper delay. I can set delay only in factors of 10 ms. I can't change the HZ value and the function cant sleep also.
Kindly suggest solution to this problem.
Thanks
It seems your system HZ value is set to 100.
If you wish to suspend execution for a period of time in a resolution lower then the system HZ, you need to use high resolution timers (which use nsec resolution, not jiffies) supported in your board and enabled in the kernel. See here for the interface of how to use them: http://lwn.net/Articles/167897/
So, either change the system HZ to 1000 and get a jiffie resolution of 1 msec or use a high resolution timer.
You can't sleep for exactly 16ms. You can sleep for at least 16ms, but not 16ms. That's not the way Linux (or any other desktop OS) works - they're not realtime OSes and they are scheduled in a non-deterministic manner and there's nothing you can do about it.
Whatever you're trying to do, you'll have to go about it another way. With what little info you've provided, all I can say is that what you're trying to do can't be done.
I need a timer tick with 1ms resolution under linux. It is used to increment a timer value that in turn is used to see if various Events should be triggered. The POSIX timerfd_create is not an option because of the glibc requirement. I tried timer_create and timer_settimer, but the best I get from them is a 10ms resolution, smaller values seem to default to 10ms resolution. Getittimer and setitimer have a 10 ms resolution according to the manpage.
The only way to do this timer I can currently think of is to use clock_gettime with CLOCK_MONOTONIC in my main loop an test if a ms has passed, and if so to increase the counter (and then check if the various Events should fire).
Is there a better way to do this than to constantly query in the main loop? What is the recommended solution to this?
The language I am using is plain old c
Update
I am using a 2.6.26 Kernel. I know you can have it interrupt at 1kHz, and the POSIX timer_* functions then can be programmed to up to 1ms but that seems not to be reliable and I don't want to use that, because it may need a new kernel on some Systems. Some stock Kernel seem to still have the 100Hz configured. And I would need to detect that. The application may be run on something else than my System :)
I can not sleep for 1ms because there may be network events I have to react to.
How I resolved it
Since it is not that important I simply declared that the global timer has a 100ms resolution. All events using their own timer have to set at least 100ms for timer expiration. I was more or less wondering if there would be a better way, hence the question.
Why I accepted the answer
I think the answer from freespace best described why it is not really possible without a realtime Linux System.
Polling in the main loop isn't an answer either - your process might not get much CPU time, so more than 10ms will elapse before your code gets to run, rendering it moot.
10ms is about the standard timer resolution for most non-realtime operating systems (RTOS). But it is moot in a non-RTOS - the behaviour of the scheduler and dispatcher is going to greatly influence how quickly you can respond to a timer expiring. For example even suppose you had a sub 10ms resolution timer, you can't respond to the timer expiring if your code isn't running. Since you can't predict when your code is going to run, you can't respond to timer expiration accurately.
There is of course realtime linux kernels, see http://www.linuxdevices.com/articles/AT8073314981.html for a list. A RTOS offers facilities whereby you can get soft or hard guarantees about when your code is going to run. This is about the only way to reliably and accurately respond to timers expiring etc.
To get 1ms resolution timers do what libevent does.
Organize your timers into a min-heap, that is, the top of the heap is the timer with the earliest expiry (absolute) time (a rb-tree would also work but with more overhead). Before calling select() or epoll() in your main event loop calculate the delta in milliseconds between the expiry time of the earliest timer and now. Use this delta as the timeout to select(). select() and epoll() timeouts have 1ms resolution.
I've got a timer resolution test that uses the mechanism explained above (but not libevent). The test measures the difference between the desired timer expiry time and its actual expiry of 1ms, 5ms and 10ms timers:
1000 deviation samples of 1msec timer: min= -246115nsec max= 1143471nsec median= -70775nsec avg= 901nsec stddev= 45570nsec
1000 deviation samples of 5msec timer: min= -265280nsec max= 256260nsec median= -252363nsec avg= -195nsec stddev= 30933nsec
1000 deviation samples of 10msec timer: min= -273119nsec max= 274045nsec median= 103471nsec avg= -179nsec stddev= 31228nsec
1000 deviation samples of 1msec timer: min= -144930nsec max= 1052379nsec median= -109322nsec avg= 1000nsec stddev= 43545nsec
1000 deviation samples of 5msec timer: min= -1229446nsec max= 1230399nsec median= 1222761nsec avg= 724nsec stddev= 254466nsec
1000 deviation samples of 10msec timer: min= -1227580nsec max= 1227734nsec median= 47328nsec avg= 745nsec stddev= 173834nsec
1000 deviation samples of 1msec timer: min= -222672nsec max= 228907nsec median= 63635nsec avg= 22nsec stddev= 29410nsec
1000 deviation samples of 5msec timer: min= -1302808nsec max= 1270006nsec median= 1251949nsec avg= -222nsec stddev= 345944nsec
1000 deviation samples of 10msec timer: min= -1297724nsec max= 1298269nsec median= 1254351nsec avg= -225nsec stddev= 374717nsec
The test ran as a real-time process on Fedora 13 kernel 2.6.34, the best achieved precision of 1ms timer was avg=22nsec stddev=29410nsec.
I'm not sure it's the best solution, but you might consider writing a small kernel module that uses the kernel high-res timers to do timing. Basically, you'd create a device file for which reads would only return on 1ms intervals.
An example of this type of approach is used in the Asterisk PBX, via the ztdummy module. If you google for ztdummy you can find the code that does this.
I think you'll have trouble achieving 1 ms precision with standard Linux even with constant querying in the main loop, because the kernel does not ensure your application will get CPU all the time. For example, you can be put to sleep for dozens of milliseconds because of preemptive multitasking and there's little you can do about it.
You might want to look into Real-Time Linux.
If you are targeting x86 platform you should check HPET timers. This is hardware timer with large precision. It must be supported by your motherbord (right now all of them support it) and your kernel should contains driver for it as well. I have used it few times without any problems and was able to achieve much better resolution than 1ms.
Here is some documentation and examples:
http://www.kernel.org/doc/Documentation/timers/hpet.txt
http://www.kernel.org/doc/Documentation/timers/hpet_example.c
http://fpmurphy.blogspot.com/2009/07/linux-hpet-support.html
I seem to recall getting ok results with gettimeofday/usleep based polling -- I wasn't needing 1000 timers a second or anything, but I was needing good accuracy with the timing for ticks I did need -- my app was a MIDI drum machine controller, and I seem to remember getting sub-millisecond accuracy, which you need for a drum machine if you don't want it to sound like a very bad drummer (esp. counting MIDI's built-in latencies) -- iirc (it was 2005 so my memory is a bit fuzzy) I was getting within 200 microseconds of target times with usleep.
However, I was not running much else on the system. If you have a controlled environment you might be able to get away with a solution like that. If there's more going on the system (watch cron firing up updatedb, etc.) then things may fall apart.
Are you running on a Linux 2.4 kernel?
From VMware KB article #1420 (http://kb.vmware.com/kb/1420).
Linux guest operating systems keep
time by counting timer interrupts.
Unpatched 2.4 and earlier kernels
program the virtual system timer to
request clock interrupts at 100Hz (100
interrupts per second). 2.6 kernels,
on the other hand, request interrupts
at 1000Hz - ten times as often. Some
2.4 kernels modified by distribution vendors to contain 2.6 features also
request 1000Hz interrupts, or in some
cases, interrupts at other rates, such
as 512Hz.
There is ktimer patch for linux kernel:
http://lwn.net/Articles/167897/
http://www.kernel.org/pub/linux/kernel/projects/rt/
HTH
First, get the kernel source and compile it with an adjusted HZ parameter.
If HZ=1000, timer interrupts 1000 times per seconds. It is ok to use HZ=1000 for an i386 machine.
On an embedded machine, HZ might be limited to 100 or 200.
For good operation, PREEMPT_KERNEL option should be on. There are
kernels which does not support this option properly. You can check them out by
searching.
Recent kernels, i.e. 2.6.35.10, supports NO_HZ options, which turns
on dynamic ticks. This means that there will be no timer ticks when in idle,
but a timer tick will be generated at the specified moment.
There is a RT patch to the kernel, but hardware support is very limited.
Generally RTAI is an all killer solution to your problem, but its
hardware support is very limited. However, good CNC controllers, like
emc2, use RTAI for their clocking, maybe 5000 Hz, but it can be
hard work to install it.
If you can, you could add hardware to generate pulses. That would make
a system which can be adapted to any OS version.
You don't need an RTOS for a simple real time application. All modern processors have General Purpose timers. Get a datasheet for whatever target CPU you are working on. Look in the kernel source, under the arch directory you will find processor specific source how to handle these timers.
There are two approaches you can take with this:
1) Your application is ONLY running your state machine, and nothing else. Linux is simply your "boot loader." Create a kernel object which installs a character device. On insertion into the kernel, set up your GP Timer to run continuously. You know the frequency it's operating at. Now, in the kernel, explicitly disable your watchdog. Now disable interrupts (hardware AND software) On a single-cpu Linux kernel, calling spin_lock() will accomplish this (never let go of it.) The CPU is YOURS. Busy loop, checking the value of the GPT until the required # of ticks have passed, when they have, set a value for the next timeout and enter your processing loop. Just make sure that the burst time for your code is under 1ms
2) A 2nd option. This assumes you are running a preemptive Linux kernel. Set up an unused a GPT along side your running OS. Now, set up an interrupt to fire some configurable margin BEFORE your 1ms timeout happens (say 50-75 uSec.) When the interrupt fires, you will immediately disable interrupts and spin waiting for 1ms window to occur, then entering your state machine and subsequently enabling interrupts on your wait OUT. This accounts for the fact that you are cooperating with OTHER things in the kernel which disable interrupts. This ASSUMES that there is no other kernel activity which locks out interrupts for a long time (more than 100us.) Now, you can MEASURE the accuracy of your firing event and make the window larger until it meets your need.
If instead you are trying to learn how RTOS's work...or if you are trying to solve a control problem with more than one real-time responsibility...then use an RTOS.
Can you at least use nanosleep in your loop to sleep for 1ms? Or is that a glibc thing?
Update: Never mind, I see from the man page "it can take up to 10 ms longer than specified until the process becomes runnable again"
What about using "/dev/rtc0" (or "/dev/rtc") device and its related ioctl() interface? I think it offers an accurate timer counter. It is not possible to set the rate just to 1 ms, but to a close value or 1/1024sec (1024Hz), or to a higher frequency, like 8192Hz.