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<chapter id="bbv-manual" xreflabel="BBV">
<title>BBV: an experimental basic block vector generation tool</title>
<para>To use this tool, you must specify
<option>--tool=exp-bbv</option> on the Valgrind
command line.</para>
<sect1 id="bbv-manual.overview" xreflabel="Overview">
A basic block is a linear section of code with one entry point and one exit
point. A <emphasis>basic block vector</emphasis> (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
<ulink url="">SimPoint</ulink>
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.
<sect1 id="bbv-manual.quickstart" xreflabel="Quick Start">
<title>Using Basic Block Vectors to create SimPoints</title>
To quickly create a basic block vector file, you will call Valgrind
like this:
<programlisting>valgrind --tool=exp-bbv /bin/ls</programlisting>
In this case we are running on <filename>/bin/ls</filename>,
but this can be any program. By default a file called
<computeroutput>bb.out.PID</computeroutput> 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
<ulink url="">SimPoint webpage</ulink>.
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]]></programlisting>
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
and <computeroutput>results.weights</computeroutput> 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.
<sect1 id="bbv-manual.usage" xreflabel="BBV Command-line Options">
<title>BBV Command-line Options</title>
<para> BBV-specific command-line options are:</para>
<!-- start of xi:include in the manpage -->
<variablelist id="bbv.opts.list">
<varlistentry id="" xreflabel="--bb-out-file">
<option><![CDATA[--bb-out-file=<name> [default: bb.out.%p] ]]></option>
This option selects the name of the basic block vector file. The
<option>%p</option> and <option>%q</option> 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
<option><xref linkend="opt.log-file"/></option>.
<varlistentry id="opt.pc-out-file" xreflabel="--pc-out-file">
<option><![CDATA[--pc-out-file=<name> [default: pc.out.%p] ]]></option>
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
<option>%p</option> and <option>%q</option> 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
<option><xref linkend="opt.log-file"/></option>.
<varlistentry id="opt.interval-size" xreflabel="--interval-size">
<option><![CDATA[--interval-size=<number> [default: 100000000] ]]></option>
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.)
<varlistentry id="opt.instr-count-only" xreflabel="--instr-count-only">
<option><![CDATA[--instr-count-only [default: no] ]]></option>
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.
<!-- end of xi:include in the manpage -->
<sect1 id="bbv-manual.fileformat" xreflabel="BBV File Format">
<title>Basic Block Vector File Format</title>
The Basic Block Vector is dumped at fixed intervals. This
is commonly done every 100 million instructions; the
<option>--interval-size</option> 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]]></programlisting>
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.
<sect1 id="bbv-manual.implementation" xreflabel="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
option to Valgrind which forces a more basic-block-like
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
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.
<sect1 id="bbv-manual.threadsupport" xreflabel="BBV Threaded Support">
<title>Threaded Executable Support</title>
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
<sect1 id="bbv-manual.validation" xreflabel="BBV 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.
<sect1 id="bbv-manual.performance" xreflabel="BBV 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).