One could use a profiler, but why not just halt the program?

Question

If something is making a single-thread program take, say, 10 times as long as it should, you could run a profiler on it. You could also just halt it with a "pause" button, and you'll see exactly what it's doing. Even if it's only 10% slower than it should be, if you halt it more times, before long you'll see it repeatedly doing the unnecessary thing. Usually the problem is a function call somewhere in the middle of the stack that isn't really needed. This doesn't measure the problem, but it sure does find it.

Edit: The objections mostly assume that you only take 1 sample. If you're serious, take 10. Any line of code causing some percentage of wastage, like 40%, will appear on the stack on that fraction of samples, on average. Bottlenecks (in single-thread code) can't hide from it.

Answer 1

Doing this manually can't really be called "quick" or "effective", but there are several profiling tools which do this automatically; also known as statistical profiling.

Answer 2

Because sometimes it works, and sometimes it gives you completely wrong answers. A profiler has a far better record of finding the right answer, and it usually gets there faster.

Answer 3

What if the program is in production and being used at the same time by paying clients or colleagues. Profiler allows you to observe without interferring (as much, because of course it will have a little hit too as per the Heisenberg principle)

Profiling can also give you much richer and more detailed accurate reports. This will be quicker in the long run.

Answer 4

EDIT 2008/11/25: OK, Vineet's response has finally made me see what the issue is here. Better late than never.

Somehow the idea got loose in the land that performance problems are found by measuring performance. That is confusing means with ends. Somehow I avoided this by single-stepping entire programs long ago. I did not berate myself for slowing it down to human speed. I was trying to see if it was doing wrong or unnecessary things. That's how to make software fast - find and remove unnecessary operations.

Nobody has the patience for single-stepping these days, but the next best thing is to pick a number of cycles at random and ask what their reasons are. (That's what the call stack can often tell you.) If a good percentage of them don't have good reasons, you can do something about it.

It's harder these days, what with threading and asynchrony, but that's how I tune software - by finding unnecessary cycles. Not by seeing how fast it is - I do that at the end.

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Here's why sampling the call stack cannot give a wrong answer, and why not many samples are needed.

During the interval of interest, when the program is taking more time than you would like, the call stack exists continuously, even when you're not sampling it.

• If an instruction I is on the call stack for fraction P(I) of that time, removing it from the program, if you could, would save exactly that much. If this isn't obvious, give it a bit of thought.

If the instruction shows up on M = 2 or more samples, out of N, its P(I) is approximately M/N, and is definitely significant.

The only way you can fail to see the instruction is to magically time all your samples for when the instruction is not on the call stack. The simple fact that it is present for a fraction of the time is what exposes it to your probes.

So the process of performance tuning is a simple matter of picking off instructions (mostly function call instructions) that raise their heads by turning up on multiple samples of the call stack. Those are the tall trees in the forest.

Notice that we don't have to care about the call graph, or how long functions take, or how many times they are called, or recursion.

I'm against obfuscation, not against profilers. They give you lots of statistics, but most don't give P(I), and most users don't realize that that's what matters.

You can talk about forests and trees, but for any performance problem that you can fix by modifying code, you need to modify instructions, specifically instructions with high P(I). So you need to know where those are, preferably without playing Sherlock Holmes. Stack sampling tells you exactly where they are.

This technique is harder to employ in multi-thread, event-driven, or systems in production. That's where profilers, if they would report P(I), could really help.

Answer 5

There's a difference between things that programmers actually do, and things that they recommend others do.

I know of lots of programmers (myself included) that actually use this method. It only really helps to find the most obvious of performance problems, but it's quick and dirty and it works.

But I wouldn't really tell other programmers to do it, because it would take me too long to explain all the caveats. It's far too easy to make an inaccurate conclusion based on this method, and there are many areas where it just doesn't work at all. (for example, that method doesn't reveal any code that is triggered by user input).

So just like using lie detectors in court, or the "goto" statement, we just don't recommend that you do it, even though they all have their uses.

Answer 6

Stepping through code is great for seeing the nitty-gritty details and troubleshooting algorithms. It's sortof like looking at a tree really up close and following each vein of bark and branch individually.

Profiling lets you see the big picture, and quickly identify trouble points -- sortof like taking a step backwards and looking at the whole forest and noticing the tallest trees. By sorting your function calls by length of execution time, you can quickly identify the areas that are the trouble points.

Answer 7

If we take the question "Why isn't it better known?" than the answer is going to be subjective. Presumably the reason why it is not better known is because profiling provides a long term solution rather than a current problem solution. It isn't effective for multi-threaded applications and isn't effective for applications like games which spend a significant portion of its time rendering.

Furthermore, in single threaded applications if you have a method that you expect to the consume the most run time and you want to reduce the run-time of all other methods then it is going to be harder to determine which secondary methods to focus your efforts upon first.

Your process for profiling is an acceptable method that can and does work but profiling provides you with more information and has the benefit of showing you more detailed performance improvements and regressions.

If you have well instrumented code than you can examine more than just the how long a particular method; you can see all the methods.

With profiling:

• You can then rerun your scenario after each change to determine the degree of performance improvement/regression.

• You can profile the code on different hardware configurations to determine if your production hardware is going to be sufficient.

• You can profile the code under load and stress testing scenarios to determine how the volume of information impacts performance

• You can make it easier for junior developers to visualise the impacts of their changes to your code because they can re-profile the code in six months time while your off at the beach or the pub, or both. Beach-pub, ftw.

Profiling is given more weight because enterprise code should always have some degree of profiling because of the benefits it gives to the organisation of an extended period of time. The more important the code the more profiling and testing you do.

Your approach is valid and is another item is the toolbox of the developer. It just gets outweighed by profiling.

Answer 8

These must be some trivial examples that you are working with to get useful results with your method. I can't think of a project where profiling was useful (by whatever method) that would have gotten decent results with your "quick and effective" method. The time it takes to start and stop some applications already puts your assertion of "quick" to shame.

Why anyone would advocate making this sampling manual rather than automated is mind boggling. It is also mind-boggling that you continue to defend this idea.

Again, with non-trivial programs the method you advocate is useless.

EDIT: Regarding "why isn't it better known"?

In my experience code reviews avoid poor quality code and algorithms, and profiling would find these as well. If you wish to continue with your method that is great - but I think for most of the professional community this is so far down on the list of things to try that it will never get positive reinforcement as a good use of time.

It appears to be quite inaccurate with small sample sets and to get large sample sets would take lots of time that would have been better spent with other useful activities.

Answer 9

I've typically used it on real-time programs that were overrunning their timeslice. You can't manually stop and restart code that has to run 60 times every second.

I've also used it to track down the bottleneck in a compiler I had written. You wouldn't want to try to break such a program manually, because you really have no way of knowing if you are breaking at the spot where the bottlenck is, or just at the spot after the bottleneck when the OS is allowed back in to stop it. Also, what if the major bottleneck is something you can't do anything about, but you'd like to get rid of all the other largeish bottlenecks in the system? How to you prioritize which bottlenecks to attack first, when you don't have good data on where they all are, and what their relative impact each is?

Answer 10

I used this method for C64 Basic many years ago. Surprising how well it works.

Answer 11

The larger your program gets, the more useful a profiler will be. If you need to optimize a program which contains thousands of conditional branches, a profiler can be indispensible. Feed in your largest sample of test data, and when it's done import the profiling data into Excel. Then you check your assumptions about likely hot spots against the actual data. There are always surprises.

Answer 12

Hitting the pause button during the execution of a program in "debug" mode might not provide the right data to perform any performance optimizations. To put it bluntly, it is a crude form of profiling.

If you must avoid using a profiler, a better bet is to use a logger, and then apply a slowdown factor to "guesstimate" where the real problem is. Profilers however, are better tools for guesstimating.

The reason why hitting the pause button in debug mode, may not give a real picture of application behavior is because debuggers introduce additional executable code that can slowdown certain parts of the application. One can refer to Mike Stall's blog post on possible reasons for application slowdown in a debugging environment. The post sheds light on certain reasons like too many breakpoints,creation of exception objects, unoptimized code etc. The part about unoptimized code is important - the "debug" mode will result in a lot of optimizations (usually code in-lining and re-ordering) being thrown out of the window, to enable the debug host (the process running your code) and the IDE to synchronize code execution. Therefore, hitting pause repeatedly in "debug" mode might be a bad idea.

Answer 13

On java servers it's always been a neat trick to do 2-3 quick ctrl-breaks in a row and get 2-3 threaddumps of all running threads. Simply looking at where all the threads "are" may give you extremely quick pinpoint to where your performance problems are.

This technique can reveal more performance problems in 2 minutes than any other technique I know of.

Answer 14

Sampling profilers are only useful when

1. You are monitoring a runtime with a small number of threads. Preferably one.
2. The call stack depth of each thread is relatively small (to reduce the incredible overhead in collection a sample).
3. You are only concerned about wall clock time and not other meters or resource bottlenecks.
4. You have not instrumented the code for management & monitoring purposes (hence the stack dump requests)
5. You mistakenly believe removing a stack frame is an effective performance improvement strategy whether the inherent costs (excluding callees) are practically zero or not
6. You can't be bothered to learn how to apply software performance engineering day-to-day in your job
7. ....

http://williamlouth.wordpress.com/2009/01/16/profiling-sampling-versus-execution-part-2/

William

Answer 15

Mike thanks for pointing out the disablement of comments on my blog. Somehow the automatic comment closure setting on the wordpress discussion page is set 1 day.

Answer 16

Stack trace snapshots only allow you to see stroboscopic x-rays of your application. You may require more accumulated knowledge which a profiler may give you.

The trick is knowing your tools well and choose the best for the job at hand.

Answer 17

I'm surprised by the religous tone on both sides.

Profiling is great, and certainly is a more refined and precise when you can do it. Sometimes you can't, and it's nice to have a trusty back-up. The pause technique is like the manual screwdriver you use when your power tool is too far away or the bateries have run-down.

Here is a short true story. An app (kind of a batch proccessing task) had been running fine in production for six months, suddenly ops is calling developers because it is going "too slow". They aren't going to let us attach a sampling profiler in production! You have to work with the tools already installed. Without stopping the production process, just using Process Explorer, (which ops had already installed on the machine) we could see a snapshot of a thread's stack. You can glance at the top of the stack, dismiss it with the enter key and get another snapshot with another mouse click. You can easily get a sample every second or so.

It doesn't take long to see if the top of the stack is most often in the database client library dll (waiting on the datase), or in another system dll (waiting for a system operation), or actually in some method of the applicaiton itself. In this case, if I remember right, we quickly noticed that 8 times in 10 the app was in a system .dll call reading or writing a network file. Sure enough recent "upgrades" had changed the performacne characteristics of a file share. Without a quick and dirty and (sysadmin sanctioned) approach to see what the app was doing in production, we would have spent far more time trying to measure the issue, than correcting the issue.

On the other hand, when performance requirements move beyond "good enough" to really pushing the envelope, a profiler becomes essential so that you can try to shave cycles from all of your closely-tied top-ten or twenty hot spots. Even if you are just trying to hold to a moderate performance requirement durring a project, when you can get the right tools lined-up to help you measure and test, and even get them integrated into your automated test process it can be fantasticly helpful.

But when the power is out (so to speak) and the batteries are dead, it's nice know how to use that manual screwdriver.

So the direct answer: Know what you can learn from halting the program, but don't be afraid of precision tools either. Most importantly know which jobs call for which tools.