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Beyond Stack Sampling: C++ Profilers

由 旧巷老猫 提交于 2019-11-27 02:20:54

First:

Time sampling profilers are more robust than CPU sampling profilers. I'm not extremely familiar with Windows development tools so I can't say which ones are which. Most profilers are CPU sampling.

A CPU sampling profiler grabs a stack trace every N instructions.
This technique will reveal portions of your code that are CPU bound. Which is awesome if that is the bottle neck in your application. Not so great if your application threads spend most of their time fighting over a mutex.

A time sampling profiler grabs a stack trace every N microseconds.
This technique will zero in on "slow" code. Whether the cause is CPU bound, blocking IO bound, mutex bound, or cache thrashing sections of code. In short what ever piece of code is slowing your application will standout.

So use a time sampling profiler if at all possible especially when profiling threaded code.

Second:

Sampling profilers generate gobs of data. The data is extremely useful, but there is often too much to be easily useful. A profile data visualizer helps tremendously here. The best tool I've found for profile data visualization is gprof2dot. Don't let the name fool you, it handles all kinds of sampling profiler output (AQtime, Sleepy, XPerf, etc). Once the visualization has pointed out the offending function(s), jump back to the raw profile data to get better hints on what the real cause is.

The gprof2dot tool generates a dot graph description that you then feed into a graphviz tool. The output is basically a callgraph with functions color coded by their impact on the application.

A few hints to get gprof2dot to generate nice output.

  • I use a --skew of 0.001 on my graphs so I can easily see the hot code paths. Otherwise the int main() dominates the graph.
  • If you're doing anything crazy with C++ templates you'll probably want to add --strip. This is especially true with Boost.
  • I use OProfile to generate my sampling data. To get good output I need configure it to load the debug symbols from my 3rd party and system libraries. Be sure to do the same, otherwise you'll see that CRT is taking 20% of your application's time when what's really going on is malloc is trashing the heap and eating up 15%.

What happened when you tried random pausing? I use it all the time on a monster app. You said it did not give enough information, and you've suggested you need high resolution. Sometimes people need a little help in understanding how to use it.

What I do, under VS, is configure the stack display so it doesn't show me the function arguments, because that makes the stack display totally unreadable, IMO.

Then I take about 10 samples by hitting "pause" during the time it's making me wait. I use ^A, ^C, and ^V to copy them into notepad, for reference. Then I study each one, to try to figure out what it was in the process of trying to accomplish at that time.

If it was trying to accomplish something on 2 or more samples, and that thing is not strictly necessary, then I've found a live problem, and I know roughly how much fixing it will save.

There are things you don't really need to know, like precise percents are not important, and what goes on inside 3rd-party code is not important, because you can't do anything about those. What you can do something about is the rich set of call-points in code you can modify displayed on each stack sample. That's your happy hunting ground.

Examples of the kinds of things I find:

  • During startup, it can be about 30 layers deep, in the process of trying to extract internationalized character strings from DLL resources. If the actual strings are examined, it can easily turn out that the strings don't really need to be internationalized, like they are strings the user never actually sees.

  • During normal usage, some code innocently sets a Modified property in some object. That object comes from a super-class that captures the change and triggers notifications that ripple throughout the entire data structure, manipulating the UI, creating and desroying obects in ways hard to foresee. This can happen a lot - the unexpected consequences of notifications.

  • Filling in a worksheet row-by-row, cell-by-cell. It turns out if you build the row all at once, from an array of values, it's a lot faster.

P.S. If you're multi-threaded, when you pause it, all threads pause. Take a look at the call stack of each thread. Chances are, only one of them is the real culprit, and the others are idling.

I've had some success with AMD CodeAnalyst.

Do you have an MFC OnIdle function? In the past I had a near real-time app I had to fix that was dropping serial packets when set at 19.2K speed which a PentiumD should have been able to keep up with. The OnIdle function was what was killing things. I'm not sure if QT has that concept, but I'd check for that too.

Re the VS Profiler -- if it's generating such large files, perhaps your sampling interval is too frequent? Try lowering it, as you probably have enough samples anyway.

And ideally, make sure you're not collecting samples until you're actually exercising the problem area. So start with collection paused, get your program to do its "slow activity", then start collection. You only need at most 20 seconds of collection. Stop collection after this.

This should help reduce your sample file sizes, and only capture what is necessary for your analysis.

I have successfully used PurifyPlus for Windows. Although it is not cheap, IBM provides a trial version that is slightly crippled. All you need for profiling with quantify are pdb files and linking with /FIXED:NO. Only drawback: No support for Win7/64.

Easyprofiler - I haven't seen it mentioned here yet so not sure if you've looked at it already. It takes a slightly different approach in how it gathers metric data. A drawback to using its compile-time profile approach is you have to make changes to the code-base. Thus you'll need to have some idea of where the slow might be and insert profiling code there.

Going by your latest comments though, it sounds like you're at least making some headway. Perhaps this tool might provide some useful metrics for you. If nothing else it has some really purdy charts and pictures :P

Two more tool suggestions.

Luke Stackwalker has a cute name (even if it's trying a bit hard for my taste), it won't cost you anything, and you get the source code. It claims to support multi threaded programs, too. So it is surely worth a spin.

http://lukestackwalker.sourceforge.net/

Also Glowcode, which I've had pointed out to me as worth using:

http://www.glowcode.com/

Unfortunately I haven't done any PC work for a while, so I haven't tried either of these. I hope the suggestions are of help anyway.

Checkout XPerf

This is free, non-invasive and extensible profiler offered by MS. It was developed by Microsoft to profile Windows.

If you're suspicious of the event loop, could overriding QCoreApplication::notify() and dosome manual profiling (one or two maps of senders/events to counts/time)?

I'm thinking that you first log the frequency of event types, then examine those events more carefully (which object sends it, what does it contain, etc). Signals across threads are queued implicitly, so they end up in the event loop (as well explicit queued connections too, obviously).

We've done it to trap and report exceptions in our event handlers, so really, every event goes through there.

Just an idea.

Edit: I see now you mentioned this in your first post. Dammit, I never thought I'd be that guy.

You can use Pin to instrument your code with finer granularity. I think Pin would let you create a tool to count how many times you enter a function or how many clockticks you spend there, roughly emulating something like VTune or CodeAnalyst. Then you could strip down which functions get instrumented until your timing issues go away.

I can tell you what I use everyday.

a) AMD Code Analyst

  • It is easy, and it will give you a quick overview of what is happening. It will be ok for most of the time.
  • With AMD CPUs, it will tell you info about the cpu pipeline, but you only need this only if you have heavy loops, like in graphic engines, video codecs, etc.

b) VTune.

  • It is very well integrated in vs2008

  • after you know the hotspots, you need to sample not only time, but other things like cache misses, and memory usage. This is very important. Setup a sampling session, and edit the properties. I always sample for time, memory read/write, and cache misses (three different runs)

But more than the tool, you need to get experience with profiling. And that means understanding how the CPU/Memory/PCI works... so, this is my 3rd option

c) Unit testing

This is very important if you are developing a big application that needs huge performance. If you cannot split the app in some pieces, it will be difficult to track cpu usage. I dont test all the cases and classes, but I have hardcoded executions and input files with important features.

My advice is using random sampling in several small tests, and try to standardise a profile strategy.

MOnsDaR

I just finished the first usable version of CxxProf, a portable manual instrumented profiling library for C++.

It fulfills the following goals:

  • Easy integration
  • Easily remove the lib during compile time
  • Easily remove the lib during runtime
  • Support for multithreaded applications
  • Support for distributed systems
  • Keep impact on a minimum

These points were ripped from the project wiki, have a look there for more details.

Disclaimer: Im the main developer of CxxProf

I use xperf/ETW for all of my profiling needs. It has a steep learning curve but is incredibly powerful. If you are profiling on Windows then you must know xperf. I frequently use this profiler to find performance problems in my code and in other people's code.

In the configuration that I use it:

  • xperf grabs CPU samples from every core that is executing code every ms. The sampling rate can be increased to 8 KHz and the samples include user-mode and kernel code. This allows finding out what a thread is doing while it is running
  • xperf records every context switch (allowing for perfect reconstruction of how much time each thread uses), plus call stacks for when threads are switched in, plus call stacks for what thread readied another thread, allowing tracing of wait chains and finding out why a thread is not running
  • xperf records all file I/O from all processes
  • xperf records all disk I/O from all processes
  • xperf records what window is active, the CPU frequency, CPU power state, UI delays, etc.
  • xperf can also record all heap allocations from one process, all virtual allocations from all processes, and much more.

That's a lot of data, all on one timeline, for all processes. No other profiler on Windows can do that.

I have blogged extensively about how to use xperf/ETW. These blog posts, and some professionally quality training videos, can be found here: http://randomascii.wordpress.com/2014/08/19/etw-training-videos-available-now/

If you want to find out what might happen if you don't use xperf read these blog posts: http://randomascii.wordpress.com/category/investigative-reporting/ These are tales of performance problems I have found in other people's code, that should have been found by the developers. This includes mshtml.dll being loaded into the VC++ compiler, a denial of service in VC++'s find-in-files, thermal throttling in a surprising number of customer machines, slow single-stepping in Visual Studio, a 4 GB allocation in a hard-disk driver, a powerpoint performance bug, and more.

Kuba Ober

Just to throw it out, even though it's not a full-blown profiler: if all you're after is hung event loops that take long processing an event, an ad-hoc tool is simple matter in Qt. That approach could be easily expanded to keep track of how long did each event take to process, and what those events were, and so on. It's not a universal profiler, but an event-loop-centric one.

In Qt, all cross-thread signal-slot calls are delivered via the event loop, as are timers, network and serial port notifications, and all user interaction,. Thus, observing the event loops is a big step towards understanding where the application is spending its time.

DevPartner, originally developed by NuMega and now distributed by MicroFocus, was once the solution of choice for profiling and code analysis (memory and resource leaks for example). I haven't tried it recently, so I cannot assure you it will help you; but I once had excellent results with it, so that this is an alternative I do consider to re-install in our code quality process (they provide a 14 days trial)

though your os is win7,the programm cann't run under xp? how about profile it under xp and the result should be a hint for win7.

There are lots of profilers listed here and I've tried a few of them myself - however I ended up writing my own based on this:

http://code.google.com/p/high-performance-cplusplus-profiler/

It does of course require that you modify the code base, but it's perfect for narrowing down bottlenecks, should work on all x86s (could be a problem with multi-core boxes, i.e. it uses rdtsc, however - this is purely for indicative timing anyway - so I find it's sufficient for my needs..)

I use Orbit profiler, easy, open source and powerfull ! https://orbitprofiler.com/

标签
c++
qt
optimization
profiling
profiler
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