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13. BBV: an experimental basic block vector generation tool

13. BBV: an experimental basic block vector generation tool

To use this tool, you must specify --tool=exp-bbv on the Valgrind command line.

13.1. Overview

A basic block is a linear section of code with one entry point and one exit point. A basic block vector (BBV) is a list of all basic blocks entered during program execution, and a count of how many times each basic block was run.

BBV is a tool that generates basic block vectors for use with the SimPoint analysis tool. The SimPoint methodology enables speeding up architectural simulations by only running a small portion of a program and then extrapolating total behavior from this small portion. Most programs exhibit phase-based behavior, which means that at various times during execution a program will encounter intervals of time where the code behaves similarly to a previous interval. If you can detect these intervals and group them together, an approximation of the total program behavior can be obtained by only simulating a bare minimum number of intervals, and then scaling the results.

In computer architecture research, running a benchmark on a cycle-accurate simulator can cause slowdowns on the order of 1000 times, making it take days, weeks, or even longer to run full benchmarks. By utilizing SimPoint this can be reduced significantly, usually by 90-95%, while still retaining reasonable accuracy.

A more complete introduction to how SimPoint works can be found in the paper "Automatically Characterizing Large Scale Program Behavior" by T. Sherwood, E. Perelman, G. Hamerly, and B. Calder.

13.2. Using Basic Block Vectors to create SimPoints

To quickly create a basic block vector file, you will call Valgrind like this:

valgrind --tool=exp-bbv /bin/ls

In this case we are running on /bin/ls, but this can be any program. By default a file called bb.out.PID will be created, where PID is replaced by the process ID of the running process. This file contains the basic block vector. For long-running programs this file can be quite large, so it might be wise to compress it with gzip or some other compression program.

To create actual SimPoint results, you will need the SimPoint utility, available from the SimPoint webpage. Assuming you have downloaded SimPoint 3.2 and compiled it, create SimPoint results with a command like the following:

./SimPoint.3.2/bin/simpoint -inputVectorsGzipped \
    -loadFVFile bb.out.1234.gz \
    -k 5 -saveSimpoints results.simpts \
    -saveSimpointWeights results.weights

where bb.out.1234.gz is your compressed basic block vector file generated by BBV.

The SimPoint utility does random linear projection using 15-dimensions, then does k-mean clustering to calculate which intervals are of interest. In this example we specify 5 intervals with the -k 5 option.

The outputs from the SimPoint run are the results.simpts and results.weights files. The first holds the 5 most relevant intervals of the program. The seconds holds the weight to scale each interval by when extrapolating full-program behavior. The intervals and the weights can be used in conjunction with a simulator that supports fast-forwarding; you fast-forward to the interval of interest, collect stats for the desired interval length, then use statistics gathered in conjunction with the weights to calculate your results.

13.3. BBV Command-line Options

BBV-specific command-line options are:

--bb-out-file=<name> [default: bb.out.%p]

This option selects the name of the basic block vector file. The %p and %q format specifiers can be used to embed the process ID and/or the contents of an environment variable in the name, as is the case for the core option --log-file.

--pc-out-file=<name> [default: pc.out.%p]

This option selects the name of the PC file. This file holds program counter addresses and function name info for the various basic blocks. This can be used in conjunction with the basic block vector file to fast-forward via function names instead of just instruction counts. The %p and %q format specifiers can be used to embed the process ID and/or the contents of an environment variable in the name, as is the case for the core option --log-file.

--interval-size=<number> [default: 100000000]

This option selects the size of the interval to use. The default is 100 million instructions, which is a commonly used value. Other sizes can be used; smaller intervals can help programs with finer-grained phases. However smaller interval size can lead to accuracy issues due to warm-up effects (When fast-forwarding the various architectural features will be un-initialized, and it will take some number of instructions before they "warm up" to the state a full simulation would be at without the fast-forwarding. Large interval sizes tend to mitigate this.)

--instr-count-only [default: no]

This option tells the tool to only display instruction count totals, and to not generate the actual basic block vector file. This is useful for debugging, and for gathering instruction count info without generating the large basic block vector files.

13.4. Basic Block Vector File Format

The Basic Block Vector is dumped at fixed intervals. This is commonly done every 100 million instructions; the --interval-size option can be used to change this.

The output file looks like this:

T:45:1024 :189:99343
T:11:78573 :15:1353  :56:1
T:18:45 :12:135353 :56:78 314:4324263

Each new interval starts with a T. This is followed on the same line by a series of basic block and frequency pairs, one for each basic block that was entered during the interval. The format for each block/frequency pair is a colon, followed by a number that uniquely identifies the basic block, another colon, and then the frequency (which is the number of times the block was entered, multiplied by the number of instructions in the block). The pairs are separated from each other by a space.

The frequency count is multiplied by the number of instructions that are in the basic block, in order to weigh the count so that instructions in small basic blocks aren't counted as more important than instructions in large basic blocks.

The SimPoint program only processes lines that start with a "T". All other lines are ignored. Traditionally comments are indicated by starting a line with a "#" character. Some other BBV generation tools, such as PinPoints, generate lines beginning with letters other than "T" to indicate more information about the program being run. We do not generate these, as the SimPoint utility ignores them.

13.5. Implementation

Valgrind provides all of the information necessary to create BBV files. In the current implementation, all instructions are instrumented. This is slower (by approximately a factor of two) than a method that instruments at the basic block level, but there are some complications (especially with rep prefix detection) that make that method more difficult.

Valgrind actually provides instrumentation at a superblock level. A superblock has one entry point but unlike basic blocks can have multiple exit points. Once a branch occurs into the middle of a block, it is split into a new basic block. Because Valgrind cannot produce "true" basic blocks, the generated BBV vectors will be different than those generated by other tools. In practice this does not seem to affect the accuracy of the SimPoint results. We do internally force the --vex-guest-chase=no option to Valgrind which forces a more basic-block-like behavior.

When a superblock is run for the first time, it is instrumented with our BBV routine. A block info (bbInfo) structure is allocated which holds the various information and statistics for the block. A unique block ID is assigned to the block, and then the structure is placed into an ordered set. Then each native instruction in the block is instrumented to call an instruction counting routine with a pointer to the block info structure as an argument.

At run-time, our instruction counting routines are called once per native instruction. The relevant block info structure is accessed and the block count and total instruction count is updated. If the total instruction count overflows the interval size then we walk the ordered set, writing out the statistics for any block that was accessed in the interval, then resetting the block counters to zero.

On the x86 and amd64 architectures the counting code has extra code to handle rep-prefixed string instructions. This is because actual hardware counts a rep-prefixed instruction as one instruction, while a naive Valgrind implementation would count it as many (possibly hundreds, thousands or even millions) of instructions. We handle rep-prefixed instructions specially, in order to make the results match those obtained with hardware performance counters.

BBV also counts the fldcw instruction. This instruction is used on x86 machines in various ways; it is most commonly found when converting floating point values into integers. On Pentium 4 systems the retired instruction performance counter counts this instruction as two instructions (all other known processors only count it as one). This can affect results when using SimPoint on Pentium 4 systems. We provide the fldcw count so that users can evaluate whether it will impact their results enough to avoid using Pentium 4 machines for their experiments. It would be possible to add an option to this tool that mimics the double-counting so that the generated BBV files would be usable for experiments using hardware performance counters on Pentium 4 systems.

13.6. Threaded Executable Support

BBV supports threaded programs. When a program has multiple threads, an additional basic block vector file is created for each thread (each additional file is the specified filename with the thread number appended at the end).

There is no official method of using SimPoint with threaded workloads. The most common method is to run SimPoint on each thread's results independently, and use some method of deterministic execution to try to match the original workload. This should be possible with the current BBV.

13.7. Validation

BBV has been tested on x86, amd64, and ppc32 platforms. An earlier version of BBV was tested in detail using hardware performance counters, this work is described in a paper from the HiPEAC'08 conference, "Using Dynamic Binary Instrumentation to Generate Multi-Platform SimPoints: Methodology and Accuracy" by V.M. Weaver and S.A. McKee.

13.8. Performance

Using this program slows down execution by roughly a factor of 40 over native execution. This varies depending on the machine used and the benchmark being run. On the SPEC CPU 2000 benchmarks running on a 3.4GHz Pentium D processor, the slowdown ranges from 24x (mcf) to 340x (vortex.2).

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