Written by Gabriel Krisman Bertazi, Software Engineer at Collabora.
Dynamic profilers are tools to collect data statistics about applications while they are running, with minimal intrusion on the application being observed.
The kind of data that can be collected by profilers varies deeply, depending on the requirements of the user. For instance, one may be interested in the amount of memory used by a specific application, or maybe the number of cycles the program executed, or even how long the CPU was stuck waiting for data to be fetched from the disks. All this information is valuable when tracking performance issues, allowing the programmer to identify bottlenecks in the code, or even to learn how to tune an application to a specific environment or workload.
In fact, maximizing performance or even understanding what is slowing down your application is a real challenge on modern computer systems. A modern CPU carries so many hardware techniques to optimize performance for the most common usage case, that if an application doesn’t intentionally exploit them, or worse, if it accidentally lies in the special uncommon case, it may end up experiencing terrible results without doing anything apparently wrong.
Let’s take a quite non-obvious way of how things can go wrong, as an example.
Forcing branch mispredictions
Based on the example from here.
The code below is a good example of how non-obvious performance assessment can be. In this function, the first for loop initializes a vector of size n with random values ranging from 0 to N. We can assume the values are well distributed enough for the vector elements to be completely unsorted.
The second part of the code has a for loop nested inside another one. The outer loop, going from 0 to K, is actually a measurement trick. By executing the inner loop many times, it stresses out the performance issues in that part of the code. In this case, it helps to reduce any external factor that may affect our measurement.
The inner loop is where things get interesting. This loop crawls over the vector and decides whether the value should be accumulated in another variable, depending on whether the element is higher than N/2 or not. This is done using an if clause, which gets compiled into a conditional branch instruction, which modifies the execution flow depending on the calculated value of the condition, in this case, if vec[i] >= N/2, it will enter the if leg, otherwise it will skip it entirely.
int rand_partsum(int n)
{
int i,k;
long sum = 0;
int *vec = malloc(n * sizeof(int));
for (i = 0; i < n; i++)
vec[i] = rand()%n;
for (k = 0; k < 1000000; k++)
for (i = 0; i < n; i++)
if (vec[i] > n/2)
sum += n[i];
return sum;
}
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When executing the code above on an Intel Core i7-5500U, with a vector size of 5000 elements (N=5000), it takes an average of 29.97 seconds. Can we do any better?
One may notice that this vector is unsorted, since each element comes from a call to rand(). What if we sorted the vector before executing the second for loop? For the sake of the example, let’s say we add a call to the glibc implementation of QuickSort right after the initialization loop.
A naive guess would suggest that the algorithm got worse, because we just added a new sorting step, thus raising the complexity of the entire code. One should assume this would result on a higher execution time.
But, in fact, when executing the sorted version in the same machine, the average execution time drops to 13.20 seconds, which is a reduction of 56% in execution time. Why does adding a new step actually reduces the execution time? The fact is that pre-sorting the vector in this case, allows the cpu to do a much better job at internally optimizing the code during execution. In this case, the issue observed was a high number of branch mispredictions, which were triggered by the conditional branch that implements the if clause.
Modern CPUs have quite deep pipelines, meaning that the instruction being fetched on any given cycle is always a few instructions down the road than the instruction actually executed on that cycle. When there is a conditional branch along the way, there are two possible paths that can be followed, and the prefetch unit has no idea which one it should choose, until all the actual condition for that instruction is calculated.
The obvious choice for the Prefetch unit on such cases is to stall and wait until the execution unit decides the correct path to follow, but stalling the pipeline like this is very costly. Instead, a speculative approach can be taken by a unit called Branch Predictor, which tries to guess which path should be taken. After the condition is calculated, the CPU verifies the guessed path: if it got the prediction right, in other words, if a branch prediction hit occurs, the execution just continues without much performance impact, but if it got it wrong, the processor needs to flush the entire pipeline, go back, and restart executing the correct path. The later is called a branch prediction miss, and is also a costly operation.
In systems with a branch predictor, like any modern CPU, the predictor is usually based on the history of the particular branches. If a conditional branch usually goes a specific way, the next time it appears, the predictor will assume it will take the same route.
Back to our example code, that if condition inside the for loop does not have any specific pattern. Since the vector elements are completely random, sometimes it will enter the if leg, sometimes it will skip it entirely. That is a very hard situation for the branch predictor, who keeps guessing wrong and triggering flushes in the pipeline, which keeps delaying the application.
In the sorted version, instead, it is very easy to guess whether it should enter the if leg or not. For the first part of the vector, where the elements are mostly < N/2, the if leg will always be skipped, while for the second part, it will always enter the if leg. The branch predictor is capable of learning this pattern after a few iterations, and is able to do much better guesses about the flow, reducing the number of branch misses, thus increasing the overall performance.
Well, pointing specific issues like this is usually hard, even for a simple code like the example above. How could we be sure that the the program is hitting enough branch mispredictions to affect performance? In fact, there are always many things that could be the cause of slowness, even for a slightly more complex program.
Perf_events is an interface in the Linux kernel and a userspace tool to sample hardware and software performance counters. It allows, among many other things, to query the CPU register for the statistics of the branch predictor, i.e. the number of prediction hits and misses of a given application.
The userspace tool, known as the perf command, is available in the usual channels of common distros. In Debian, for instance, you can install it with:
sudo apt install linux-perf
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We’ll dig deeper into the perf tool later on another post, but for now, let’s use the, perf record and perf annotate commands, which allow tracing the program and annotating the source code with the time spent on each instruction, and the perf stat command, which allows to run a program and display statistics about it:
At first, we can instruct perf to instrument the program and trace its execution:
[krisman@dilma bm]$ perf record ./branch-miss.unsorted
[ perf record: Woken up 19 times to write data ]
[ perf record: Captured and wrote 4.649 MB perf.data (121346 samples) ]
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The perf record will execute the program passed as parameter and collect performance information into a new perf.data file. This file can then be passed to other perf commands. In this case, we pass it to the perf annotate command, which crawls over each address in the program and prints the number of samples that was collected while the program was executing each instruction. Instructions with a higher number of samples indicates that the program spent more time in that region, indicating that it is hot code, and a good part of the program to try to optimize. Notice that, for modern processors, the exact position is an estimation, so this information must be used with care. As a rule of thumb, one should be looking for hot regions, instead of single hot instructions.
Below is the output of perf annotate, when analyzing the function above. The output is truncated to display only the interesting parts.
[krisman@dilma bm]$ perf annotate
:
: int rand_partsum()
: {
0.00 : 74e: push %rbp
0.00 : 74f: mov %rsp,%rbp
0.00 : 752: push %rbx
0.00 : 753: sub $0x38,%rsp
0.00 : 757: mov %rsp,%rax
0.00 : 75a: mov %rax,%rbx
[...]
0.00 : 7ce: mov $0x0,%edi
0.00 : 7d3: callq 5d0 <time@plt>
0.00 : 7d8: mov %eax,%edi
0.00 : 7da: callq 5c0 <srand@plt>
: for (i = 0; i < n; i++)
0.00 : 7df: movl $0x0,-0x14(%rbp)
0.00 : 7e6: jmp 804 <main+0xb6>
: vec[i] = rand()%n;
0.00 : 7e8: callq 5e0 <rand@plt>
0.00 : 7ed: cltd
0.00 : 7ee: idivl -0x24(%rbp)
0.00 : 7f1: mov %edx,%ecx
0.00 : 7f3: mov -0x38(%rbp),%rax
0.00 : 7f7: mov -0x14(%rbp),%edx
0.00 : 7fa: movslq %edx,%rdx
0.00 : 7fd: mov %ecx,(%rax,%rdx,4)
: for (i = 0; i < n; i++)
0.00 : 800: addl $0x1,-0x14(%rbp)
0.00 : 804: mov -0x14(%rbp),%eax
0.00 : 807: cmp -0x24(%rbp),%eax
0.00 : 80a: jl 7e8 <main+0x9a>
[...]
: for (k = 0; k < 1000000; k++)
0.00 : 80c: movl $0x0,-0x18(%rbp)
0.00 : 813: jmp 85e <main+0x110>
: for (i = 0; i < n; i++)
0.01 : 815: movl $0x0,-0x14(%rbp)
0.00 : 81c: jmp 852 <main+0x104>
: if (vec[i] > n/2)
0.20 : 81e: mov -0x38(%rbp),%rax
6.47 : 822: mov -0x14(%rbp),%edx
1.94 : 825: movslq %edx,%rdx
26.86 : 828: mov (%rax,%rdx,4),%edx
0.08 : 82b: mov -0x24(%rbp),%eax
1.46 : 82e: mov %eax,%ecx
0.62 : 830: shr $0x1f,%ecx
3.82 : 833: add %ecx,%eax
0.06 : 835: sar %eax
0.70 : 837: cmp %eax,%edx
0.42 : 839: jle 84e <main+0x100>
: sum += vec[i];
9.15 : 83b: mov -0x38(%rbp),%rax
5.91 : 83f: mov -0x14(%rbp),%edx
0.26 : 842: movslq %edx,%rdx
5.87 : 845: mov (%rax,%rdx,4),%eax
2.09 : 848: cltq
9.31 : 84a: add %rax,-0x20(%rbp)
: for (i = 0; i < n; i++)
16.66 : 84e: addl $0x1,-0x14(%rbp)
6.46 : 852: mov -0x14(%rbp),%eax
0.00 : 855: cmp -0x24(%rbp),%eax
1.63 : 858: jl 81e <main+0xd0>
: for (k = 0; k < 1000000; k++)
[...]
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The first thing to notice is that the perf command tries to interleave C code with the Assembly code. This feature requires compiling the test program with -g3 to include debug information.
The number before the ‘:’ is the percentage of samples collected while the program was executing each instruction. Once again, this is not an exact information, so you should be looking for hot regions, and not specific instructions.
The first and second hunk are the function prologue, which was executed only once, and the vector initialization. According to the profiling data, there is little point in attempting to optimize them, because the execution practically didn’t spend any time on it. The third hunk is the second loop, where it spent almost all the execution time. Since that loop is where most of our samples where collected, we can assume that it is a hot region, which we can try to optimize. Also, notice that most of the samples were collected around that if leg. This is another indication that we should look into that specific code.
To find out what might be causing the slowness, we can use the perf stat command, which prints a bunch of performance counters information for the entire program. Let’s take a look at its output.
[krisman@dilma bm]$ perf stat ./branch-miss.unsorted
Performance counter stats for './branch-miss.unsorted:
29876.773720 task-clock (msec) # 1.000 CPUs utilized
25 context-switches # 0.001 K/sec
0 cpu-migrations # 0.000 K/sec
49 page-faults # 0.002 K/sec
86,685,961,134 cycles # 2.901 GHz
90,235,794,558 instructions # 1.04 insn per cycle
10,007,460,614 branches # 334.958 M/sec
1,605,231,778 branch-misses # 16.04% of all branches
29.878469405 seconds time elapsed
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Perf stat will dynamically profile the program passed in the command line and report back a number of statistics about the entire execution. In this case, let’s look at the 3 last lines in the output. The first one gives the rate of instructions executed per CPU cycle; the second line, the total number of branches executed; and the third, the percentage of those branches that resulted in a branch miss and pipeline flush.
Perf is even nice enough to put important or unexpected results in red. In this case, the last line, Branch-Misses, was unexpectedly high, thus it was displayed in red in this test.
And now, let’s profile the pre-sorted version. Look at the number of branch misses:
[krisman@dilma bm]$ perf stat ./branch-miss.sorted
Performance counter stats for './branch-miss.sorted:
14003.066457 task-clock (msec) # 0.999 CPUs utilized
175 context-switches # 0.012 K/sec
4 cpu-migrations # 0.000 K/sec
56 page-faults # 0.004 K/sec
40,178,067,584 cycles # 2.869 GHz
89,689,982,680 instructions # 2.23 insn per cycle
10,006,420,927 branches # 714.588 M/sec
2,275,488 branch-misses # 0.02% of all branches
14.020689833 seconds time elapsed
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It went down from over 16% to just 0.02% of the total branches! This is very impressive and is likely to explain the reduction in execution time. Another interesting value is the number of instructions per cycle, which more than doubled. This happens because, once we reduced the number of stalls, we make better use of the pipeline, obtaining a better instruction throughput.
Wrapping up
As demonstrated by the example above, figuring out the root cause of a program slowness is not always easy. In fact, it gets more complicated every time a new processor comes out with a bunch of shiny new optimizations.
Despite being a short example code, the branch misprediction case is still quite non-trivial for anyone not familiar with how the branch prediction mechanism works. In fact, if we just look at the algorithm, we could have concluded that adding a sort algorithm would just add more overhead to the algorithm. Thus, this example gives us a high-level view of how helpful profiling tools really are. By using just one of the several features provided by the perf tool, we were able to draw major conclusions about the program being examined.
