Suppose I have a program that runs in a given amount of time (say, three seconds). I want to run this program so that it runs n-times slower (specified on command line). How would you achieve it with (or better, without) changes to the program ?
please note that adding a sleep at the end is not a solution. The program has to run slower, not to run at full speed for the first three seconds and then do nothing for the remaining time. Also, using "nice" under unix is not a good solution either. it will run slower if other processes demand the processor, but at full speed if nothing is processor-demanding at the same time.
This is a curiosity question. Nothing serious to do related to it. The fact is that I remember 15-20 years ago games that were simply too fast to play on new processors, because they were timed with the processor clock. You had to turn off the turbo.
Let's assume the program is a C compiled program.
One idea is to write a 'ptrace runner.' ptrace is the call that allows you to implement a debugger on platforms such as Linux and Mac.
The idea is to attach to the program and then just repeatedly tell the application to run one instruction with ptrace(PTACE_SINGLESTEP). If that's not slow enough, you could add a sleep between each call to ptrace in the runner program.
I wrote a simple example on my linux box how to slow down a child process with SIGSTOP and SIGCONT signals:
#include <unistd.h>
#include <stdio.h>
#include <signal.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
void dosomething(void){
static volatile unsigned char buffer[1000000];
for(unsigned i=0;i<1000;i++) for(unsigned j=0;j<sizeof(buffer);buffer[j++]=i){;}
}
#define RUN 1
#define WAIT 1
int main(void){
int delay=0, status, pid = fork();
if( !pid ){ kill(getpid(),SIGSTOP); dosomething(); return 0; }
do{
waitpid( pid, &status, WUNTRACED | WCONTINUED );
if( WIFSTOPPED (status) ){ sleep(delay); kill(pid,SIGCONT); }
if( WIFCONTINUED(status) && WAIT ){ sleep(RUN ); kill(pid,SIGSTOP); }
delay=WAIT;
}while( !WIFEXITED(status) && !WIFSIGNALED (status) );
}
No slowdown when WAIT is zero, otherwise after every RUN seconds the parent stop the child for WAIT seconds.
Runtime results:
RUN=1 WAIT=0
---------------
real 3.905s
user 3.704s
sys 0.012s
RUN=1 WAIT=1
---------------
real 9.061s
user 3.640s
sys 0.016s
RUN=1 WAIT=2
---------------
real 13.027s
user 3.372s
sys 0.032s
cpulimit is a tool that does something like this. It works by periodically
kill -STOP and kill -CONT the process, which has the effect of it running slower (when averaged over time).
If you have DTrace you may be able to use it's chill() function. You could insert this chill at almost anyplace in a userland application and in multiple places. It's been used before to replicate race conditions seen on slower systems.
I ran some application in a virtual machine under ubuntu. It was really slow.
You could configure the virtual machine usage of the system.
You might obfuscate the situation a little further by running a virtual machine under a virtual machine under a virtual machine, ...
Related
So I have this program that continuously check until the condition is true. My problem is whenever I run it, my computer slows down because of the loop. Can anyone please suggest the best and most efficient way to do this? Thank you for your response in advance.
To illustrate my problem, here is a code that represents it:
#include <stdio.h>
#include <time.h>
#include <string.h>
#include <windows.h>
int main(void){
time_t now;
struct tm *local;
while(1){
time(&now);
local = localtime(&now);
if(local->tm_min > 55){
printf("Time:\t%d:%d:%d\n",local->tm_hour,local->tm_min,local->tm_sec);
getch();
exit(0);
}
}
return 0;
}
If polling is really what you want, or you have to use it, then you must give breath to the system by using sleep's.
So, how much to sleep in each iteration? It can be a fixed value (and if you sleep just 1 millisecond you will be stunned at how this is effective). A fixed value, say 20-30 milliseconds is good if you check for slow events like keystrokes by a real user. If, say, you are monitoring a serial port, perhaps you need lower values.
Then, depending on the application, you can also implement a variable sleep time. For example (this is a little stupid but it is just to explain): you wait for keystrokes, and sleep 30 milliseconds. Then you use your program in a pipe and discover that it is painly slow. A solution could be to set the value to sleep equal to 30 ms, but after having read a character, the value is lowered to 0 which causes the sleep to be not performed. Every time the condition fails the value is raised up to the maximum limit (20-30 milliseconds for a keyboard).
-- EDIT AFTER COMMENTS --
It has pointed out that keyboards and serial ports do not need polling, or they should not be polled. Generally speaking this is true, but it depends on the hardware and operating system (which in turn is a piece of software and, if the hardware does not support an interrupt for a given condition, even the OS would have to poll). About keyboards, for example, I thought at those little ones implemented as a matrix of buttons: some small CPUs have special facilities to generate an interrupt on any I/O change, but other don't: in that case polling is the only solution - and it is also ideal for implementing anti-bouncing (this kind of polling is not necessarily performed inside a loop).
For serial ports, it is almost true that nobody would implement one without an interrupt (to avoid polling). But even so, it is difficult to manage the incoming data in an event-driven fashion; often a flag is set, and some other part of the program, which polls that flag, will work out the message.
Event-driven programming seems easy at first, but as soon the program gets bigger the complication augments too.
There are other situations to consider, for example loops which read data from somewhere and process those data. If something else has to be done inside the loop, for example checking how much time is passed, but the reading is blocking, the reading must be implemented in a non-blocking way, and the whole loop must turn into a kind of polling for one or more conditions -unless one uses multi-threading.
Anyway, I agree that polling is evil and should only be used when necessary.
Efficiently? One way or the other you need to put your process to sleep until the condition WILL BE TRUE - then wake up and die (so to speak :-). Since your code includes windows.h I'll assume you're running on Windows and thus have the Sleep() function available.
#include <stdio.h>
#include <windows.h>
#include <time.h>
int main(void)
{
time_t now;
struct tm *local;
DWORD msecs;
time(&now);
local = localtime(&now);
/* (55 * 60000) = msecs in 55 minutes */
msecs = (55 * 60000) - ((local->tm_min * 60000) + (local->tm_sec * 1000));
if(msecs > 0)
Sleep(msecs)
return 0;
}
Is there any function in C to check if the computer is going to sleep,hibernate or locked and waking up from these state?
In msdn they provided for C#, C++ but not for C.
My OS is windows7
Like below is the code I'm using to check the time duration between starting the program and terminating it(shutting down the system will terminate the program so this way time duration can be measured).
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include<time.h>
clock_t start_time=0;
void bye (void)
{
FILE *read,*write;
write=fopen("F:\\count.txt","w");
clock_t end_time=clock();
fprintf(write,"Time: %d",(end_time-start_time)/CLOCKS_PER_SEC);
fclose(write);
}
int main (void)
{
start_time=clock();
atexit (bye);
//exit (EXIT_SUCCESS);
getch();
}
In the same way I want to check for locked/sleep/hibernate.
One possible way to wrap the c++ code(provided in the link) in c as mentioned by #ddriver
But is it not possible in C at all?
The WinAPI has generally at least the same possibilities as .NET framework. What your are asking for is the PowerManagement API.
You will have to register to receive PowerSettingNotificationEvents with the RegisterPowerSettingNotification function. Unfortunately, it is used differently for a GUI application where you give a handle to a window that will then receive a WM_POWERBROADCAST message each time the system is about to change state (one of the suspend modes or the hibernate mode), and for a non GUI (typically a service) that registers a HandlerEx callback with a dwControl parameter of SERVICE_CONTROL_POWEREVENT and a dwEventType of PBT_POWERSETTINGCHANGE.
The link you provide is about signals, emitted when power mode is changing. So, obviously, you can check when the system is about to go to sleep, or it just woke up.
As of checking if the system currently sleeps, that is simply not possible, as user code will simply not be running during deep sleep states. Maybe some platform specific, very low level BIOS API, but those are usually not public, and far from portable.
With the very simple code below, my system (Ubuntu Linux 14.04) simply crashes not even letting my mouse respond. I had to force quit with the power button. I thought Linux is a stable OS tolerable of handling such basic program errors. Did I miss something?
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <semaphore.h>
void check(int isOkay){
if(!isOkay){
printf("error\n");
abort();
}
}
int main(void){
#define n 1000000
int array[n];
sem_t blocker;
int i;
while(1){
if(!fork()){
for(i = 0; i < n; ++i){
array[i] = rand();
}
check(sem_init(&blocker, 0, 0) == 0);
check(sem_wait(&blocker) == 0);
}
}
return 0;
}
Congratulations, you've discovered the fork bomb. There are shell one-liners that can wreak the same sort of havic with a lot less typing on your part.
It is in fact possible to limit the number of processes that a user can spawn using ulimit -- see the bottom of the linked wikipedia articles for details.
A desktop install of Ubuntu is not exactly a hardened server, though. It's designed for usability first and foremost. If you need a locked down system that can't crash, there are better options.
The command ulmit -u shows the maximum number of processes that you can start. However, do not start that many processes in the background: your machine would spend time switching between processes and wouldn't get around to getting actual work done.
The linux does its job of processing your request to create a process, it is for the user to implement his code based on this limit.
The main problem here is determining the best limit. A lot of software doesn't use fork() at all, so do you set the limit to something small like 5? Some software might create a new process whenever it receives a request from network, so do you set the limit to "max. number of network packets"? If you assume most software isn't buggy, then you'd be tempted to set the limit relatively high so that correct software works properly.
The other problem is one of scheduling priorities. In a well designed system things like the GUI would be "high priority" and if it wants CPU time it'd preempt normal/lower priority work immediately. If this was the case, a massive fork bomb running at normal/lower priority would have no effect on the system's ability to respond to the user, and the user would be able to kill the fork bomb without much problem.
Sadly, for a variety of reasons, the scheduler in Linux doesn't work like that. It does support priorities, but to use them you have to be a "real time" process and have to be running as root (which is a massive security disaster). Without sane priorities, Linux assumes that every forked process is as important as everything else, and the CPU/s end up busy doing the forking and there's no CPU time left to respond to the user.
I'm working on a timing system and I'll implement a timer class.
#include <windows.h>
#include <stdio.h>
#include <time.h>
int main()
{
clock_t t1, t2;
t1 = clock();
Sleep(10);
t2 = clock();
printf("%i\n", (int)(t2 - t1));
return 0;
}
This program should print "10" but it prints "15" or "16". I need more accurate which is less than 1 ms! Suggestions? (maybe with select()'s timeout?)
NOTE: I've run this program on Windows 7 Ultimate x86. Program compiled with MinGW (C/C++) x86.
NOW I THINK >>
Sleep() is accurate to the operating system's clock interrupt rate. Which by default on Windows ticks 64 times per second. Or once every 15.625 msec, as you found out.
You can increase that rate, call timeBeginPeriod(10). Use timeEndPeriod(10) when you're done. You are still subject to normal thread scheduling latencies so you still don't have a guarantee that your thread will resume running after 10 msec. And won't when the machine is heavily loaded. Using SetThreadPriority() to boost the priority, increasing the odds that it will.
Your problem is that the "clock" only ticks 64 times per second (I've seen systems that can go up to 2000, but no higher). If you are creating a timer class, you probably want to have much higher resolution than even 2000. Use QueryPerformanceCounter to get higher resolution. See QueryPerformanceCounter and overflows for an example.
Note that if you want to sleep for very short intervals, you will have to call QueryPerformanceCounter in a tight loop.
Sleep() is not accurate in the way you want it to be.
It will cause the thread to sleep for AT LEAST the length of time you specify, but there's no guarantee that the OS will return control to your thread at exactly that specified time.
On Windows I can do:
HANDLE hCurrentProcess = GetCurrentProcess();
SetPriorityClass(hCurrentProcess, ABOVE_NORMAL_PRIORITY_CLASS);
How can I do the same thing on *nix?
Try:
#include <sys/time.h>
#include <sys/resource.h>
int main(){
setpriority(PRIO_PROCESS, 0, -20);
}
Note that you must be running as superuser for this to work.
(for more info, type 'man setpriority' at a prompt.)
If doing something like this under unix your want to (as root) chmod you task and set the s bit. Then you can change who you are running as, what your priority is, your thread scheduling, etc. at run time.
It is great as long as you are not writing a massively multithreaded app with a bug in it so that you take over a 48 CPU box and nobody can shut you down because your have each CPU spinning at 100% with all thread set to SHED_FIFO (runs to completion) running as root.
Nah .. I wouldn't be speaking from experience ....
# allain Can you lower your own process' priority without being superuser?
Sure. Be aware, however, that this is a one way street. You can't even get back to where you started. And even fairly small reductions in priority can have startlingly large effects on running time when there is significant load on the system.