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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../intro.html">Introduction</a></li><li class="chapter-item expanded "><a href="../shells/index.html"><strong aria-hidden="true">1.</strong> Shells</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../shells/zsh.html"><strong aria-hidden="true">1.1.</strong> zsh</a></li><li class="chapter-item expanded "><a href="../shells/bash.html"><strong aria-hidden="true">1.2.</strong> bash</a></li><li class="chapter-item expanded "><a href="../shells/fish.html"><strong aria-hidden="true">1.3.</strong> fish</a></li></ol></li><li class="chapter-item expanded "><a href="../cli/index.html"><strong aria-hidden="true">2.</strong> CLI foo</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../cli/awk.html"><strong aria-hidden="true">2.1.</strong> awk</a></li><li class="chapter-item expanded "><a href="../cli/sed.html"><strong aria-hidden="true">2.2.</strong> sed</a></li><li class="chapter-item expanded "><a href="../cli/column.html"><strong aria-hidden="true">2.3.</strong> column</a></li><li class="chapter-item expanded "><a href="../cli/sort.html"><strong aria-hidden="true">2.4.</strong> sort</a></li><li class="chapter-item expanded "><a href="../cli/tr.html"><strong aria-hidden="true">2.5.</strong> tr</a></li><li class="chapter-item expanded "><a href="../cli/tac.html"><strong aria-hidden="true">2.6.</strong> tac</a></li><li class="chapter-item expanded "><a href="../cli/paste.html"><strong aria-hidden="true">2.7.</strong> paste</a></li></ol></li><li class="chapter-item expanded "><a href="../tools/index.html"><strong aria-hidden="true">3.</strong> Tools</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tools/tmux.html"><strong aria-hidden="true">3.1.</strong> tmux</a></li><li class="chapter-item expanded "><a href="../tools/emacs.html"><strong aria-hidden="true">3.2.</strong> emacs</a></li><li class="chapter-item expanded 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href="../tools/qrencode.html"><strong aria-hidden="true">3.11.</strong> qrencode</a></li></ol></li><li class="chapter-item expanded "><a href="../process/index.html"><strong aria-hidden="true">4.</strong> Process management & inspection</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../process/lsof.html"><strong aria-hidden="true">4.1.</strong> lsof</a></li><li class="chapter-item expanded "><a href="../process/pidstat.html"><strong aria-hidden="true">4.2.</strong> pidstat</a></li><li class="chapter-item expanded "><a href="../process/pgrep.html"><strong aria-hidden="true">4.3.</strong> pgrep</a></li><li class="chapter-item expanded "><a href="../process/ps.html"><strong aria-hidden="true">4.4.</strong> ps</a></li><li class="chapter-item expanded "><a href="../process/pmap.html"><strong aria-hidden="true">4.5.</strong> pmap</a></li><li class="chapter-item expanded "><a href="../process/pstack.html"><strong aria-hidden="true">4.6.</strong> pstack</a></li><li class="chapter-item expanded "><a href="../process/taskset.html"><strong aria-hidden="true">4.7.</strong> taskset</a></li><li class="chapter-item expanded "><a href="../process/nice.html"><strong aria-hidden="true">4.8.</strong> nice</a></li></ol></li><li class="chapter-item expanded "><a href="../trace_profile/index.html"><strong aria-hidden="true">5.</strong> Trace and Profile</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../trace_profile/time.html"><strong aria-hidden="true">5.1.</strong> time</a></li><li class="chapter-item expanded "><a href="../trace_profile/strace.html"><strong aria-hidden="true">5.2.</strong> strace</a></li><li class="chapter-item expanded "><a href="../trace_profile/ltrace.html"><strong aria-hidden="true">5.3.</strong> ltrace</a></li><li class="chapter-item expanded "><a href="../trace_profile/perf.html"><strong aria-hidden="true">5.4.</strong> perf</a></li><li class="chapter-item expanded "><a href="../trace_profile/oprofile.html"><strong aria-hidden="true">5.5.</strong> OProfile</a></li><li class="chapter-item expanded "><a href="../trace_profile/callgrind.html"><strong aria-hidden="true">5.6.</strong> callgrind</a></li></ol></li><li class="chapter-item expanded "><a href="../debug/index.html"><strong aria-hidden="true">6.</strong> Debug</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../debug/gdb.html"><strong aria-hidden="true">6.1.</strong> gdb</a></li><li class="chapter-item expanded "><a href="../debug/gdbserver.html"><strong aria-hidden="true">6.2.</strong> gdbserver</a></li></ol></li><li class="chapter-item expanded "><a href="../binary/index.html"><strong aria-hidden="true">7.</strong> Binary</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../binary/od.html"><strong aria-hidden="true">7.1.</strong> od</a></li><li class="chapter-item expanded "><a href="../binary/xxd.html"><strong aria-hidden="true">7.2.</strong> xxd</a></li><li class="chapter-item expanded "><a href="../binary/readelf.html"><strong aria-hidden="true">7.3.</strong> readelf</a></li><li class="chapter-item expanded "><a href="../binary/objdump.html"><strong aria-hidden="true">7.4.</strong> objdump</a></li><li class="chapter-item expanded "><a href="../binary/nm.html"><strong aria-hidden="true">7.5.</strong> nm</a></li></ol></li><li class="chapter-item expanded "><a href="../development/index.html"><strong aria-hidden="true">8.</strong> Development</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/c++filt.html"><strong aria-hidden="true">8.1.</strong> c++filt</a></li><li class="chapter-item expanded "><a href="../development/c++.html"><strong aria-hidden="true">8.2.</strong> c++</a></li><li class="chapter-item expanded "><a href="../development/glibc.html"><strong aria-hidden="true">8.3.</strong> glibc</a></li><li class="chapter-item expanded "><a href="../development/gcc.html"><strong aria-hidden="true">8.4.</strong> gcc</a></li><li class="chapter-item expanded "><a href="../development/git.html"><strong aria-hidden="true">8.5.</strong> git</a></li><li class="chapter-item expanded "><a href="../development/cmake.html"><strong aria-hidden="true">8.6.</strong> cmake</a></li><li class="chapter-item expanded "><a href="../development/make.html"><strong aria-hidden="true">8.7.</strong> make</a></li><li class="chapter-item expanded "><a href="../development/ld.so.html"><strong aria-hidden="true">8.8.</strong> ld.so</a></li><li class="chapter-item expanded "><a href="../development/symbolver.html"><strong aria-hidden="true">8.9.</strong> symbol versioning</a></li><li class="chapter-item expanded "><a href="../development/python.html"><strong aria-hidden="true">8.10.</strong> python</a></li><li class="chapter-item expanded "><a href="../development/gcov.html"><strong aria-hidden="true">8.11.</strong> gcov</a></li><li class="chapter-item expanded "><a href="../development/pgo.html" class="active"><strong aria-hidden="true">8.12.</strong> pgo</a></li></ol></li><li class="chapter-item expanded "><a href="../linux/index.html"><strong aria-hidden="true">9.</strong> Linux</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../linux/systemd.html"><strong aria-hidden="true">9.1.</strong> systemd</a></li><li class="chapter-item expanded "><a href="../linux/coredump.html"><strong aria-hidden="true">9.2.</strong> coredump</a></li><li class="chapter-item expanded "><a href="../linux/ptrace_scope.html"><strong aria-hidden="true">9.3.</strong> ptrace_scope</a></li><li class="chapter-item expanded "><a href="../linux/cryptsetup.html"><strong aria-hidden="true">9.4.</strong> cryptsetup</a></li><li class="chapter-item expanded "><a href="../linux/swap.html"><strong aria-hidden="true">9.5.</strong> swap</a></li><li class="chapter-item expanded "><a href="../linux/input.html"><strong aria-hidden="true">9.6.</strong> input</a></li><li class="chapter-item expanded "><a href="../linux/acl.html"><strong aria-hidden="true">9.7.</strong> acl</a></li><li class="chapter-item expanded "><a href="../linux/zfs.html"><strong aria-hidden="true">9.8.</strong> zfs</a></li><li class="chapter-item expanded "><a href="../linux/cpufreq.html"><strong aria-hidden="true">9.9.</strong> cpufreq</a></li><li class="chapter-item expanded "><a href="../linux/cups.html"><strong aria-hidden="true">9.10.</strong> cups</a></li></ol></li><li class="chapter-item expanded "><a href="../network/index.html"><strong aria-hidden="true">10.</strong> Network</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../network/ssh.html"><strong aria-hidden="true">10.1.</strong> ssh</a></li><li class="chapter-item expanded "><a href="../network/ss.html"><strong aria-hidden="true">10.2.</strong> ss</a></li><li class="chapter-item expanded "><a href="../network/tcpdump.html"><strong aria-hidden="true">10.3.</strong> tcpdump</a></li><li class="chapter-item expanded "><a href="../network/tshark.html"><strong aria-hidden="true">10.4.</strong> tshark</a></li><li class="chapter-item expanded "><a href="../network/firewall-cmd.html"><strong aria-hidden="true">10.5.</strong> firewall-cmd</a></li><li class="chapter-item expanded "><a href="../network/nftables.html"><strong aria-hidden="true">10.6.</strong> nftables</a></li></ol></li><li class="chapter-item expanded "><a href="../web/index.html"><strong aria-hidden="true">11.</strong> Web</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../web/html.html"><strong aria-hidden="true">11.1.</strong> html</a></li><li class="chapter-item expanded "><a href="../web/css.html"><strong aria-hidden="true">11.2.</strong> css</a></li><li class="chapter-item expanded "><a href="../web/chartjs.html"><strong aria-hidden="true">11.3.</strong> chartjs</a></li></ol></li><li class="chapter-item expanded "><a href="../arch/index.html"><strong aria-hidden="true">12.</strong> Arch</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../arch/x86_64.html"><strong aria-hidden="true">12.1.</strong> x86_64</a></li><li class="chapter-item expanded "><a href="../arch/armv8.html"><strong aria-hidden="true">12.2.</strong> armv8</a></li><li class="chapter-item expanded "><a href="../arch/arm64.html"><strong aria-hidden="true">12.3.</strong> arm64</a></li><li class="chapter-item expanded "><a href="../arch/armv7.html"><strong aria-hidden="true">12.4.</strong> armv7</a></li><li class="chapter-item expanded "><a href="../arch/riscv.html"><strong aria-hidden="true">12.5.</strong> riscv</a></li></ol></li></ol>
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<h1 id="profile-guided-optimization-pgo"><a class="header" href="#profile-guided-optimization-pgo">Profile guided optimization (pgo)</a></h1>
<p><code>pgo</code> is an optimization technique to optimize a program for its usual
workload.</p>
<p>It is applied in two phases:</p>
<ol>
<li>Collect profiling data (best with representative benchmarks).</li>
<li>Optimize program based on collected profiling data.</li>
</ol>
<p>The following simple program is used as demonstrator.</p>
<pre><code class="language-c">#include <stdio.h>
#define NOINLINE __attribute__((noinline))
NOINLINE void foo() { puts("foo()"); }
NOINLINE void bar() { puts("bar()"); }
int main(int argc, char *argv[]) {
if (argc == 2) {
foo();
} else {
bar();
}
}
</code></pre>
<h2 id="clang"><a class="header" href="#clang">clang</a></h2>
<p>On the actual machine with <code>clang 15.0.7</code>, the following code is generated for
the <code>main()</code> function.</p>
<pre><code class="language-x86asm"># clang -o test test.c -O3
0000000000001160 <main>:
1160: 50 push rax
; Jump if argc != 2.
1161: 83 ff 02 cmp edi,0x2
1164: 75 09 jne 116f <main+0xf>
; foor() is on the hot path (fall-through).
1166: e8 d5 ff ff ff call 1140 <_Z3foov>
116b: 31 c0 xor eax,eax
116d: 59 pop rcx
116e: c3 ret
; bar() is on the cold path (branch).
116f: e8 dc ff ff ff call 1150 <_Z3barv>
1174: 31 c0 xor eax,eax
1176: 59 pop rcx
1177: c3 ret
</code></pre>
<p>The following shows how to compile with profiling instrumentation and how to
optimize the final program with the collected profiling data (<a href="https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization">llvm
pgo</a>).</p>
<p>The arguments to <code>./test</code> are chosen such that <code>9/10</code> runs call <code>bar()</code>, which
is currently on the <code>cold path</code>.</p>
<pre><code class="language-bash"># Compile test program with profiling instrumentation.
clang -o test test.cc -O3 -fprofile-instr-generate
# Collect profiling data from multiple runs.
for i in {0..10}; do
LLVM_PROFILE_FILE="prof.clang/%p.profraw" ./test $(seq 0 $i)
done
# Merge raw profiling data into single profile data.
llvm-profdata merge -o pgo.profdata prof.clang/*.profraw
# Optimize test program with profiling data.
clang -o test test.cc -O3 -fprofile-use=pgo.profdata
</code></pre>
<blockquote>
<p>NOTE: If <code>LLVM_PROFILE_FILE</code> is not given the profile data is written to
<code>default.profraw</code> which is re-written on each run. If the <code>LLVM_PROFILE_FILE</code>
contains a <code>%m</code> in the filename, a unique integer will be generated and
consecutive runs will update the same generated profraw file,
<code>LLVM_PROFILE_FILE</code> can specify a new file every time, however that requires
more storage in general.</p>
</blockquote>
<p>After optimizing the program with the profiling data, the <code>main()</code> function
looks as follows.</p>
<pre><code class="language-x86asm">0000000000001060 <main>:
1060: 50 push rax
; Jump if argc == 2.
1061: 83 ff 02 cmp edi,0x2
1064: 74 09 je 106f <main+0xf>
; bar() is on the hot path (fall-through).
1066: e8 e5 ff ff ff call 1050 <_Z3barv>
106b: 31 c0 xor eax,eax
106d: 59 pop rcx
106e: c3 ret
; foo() is on the cold path (branch).
106f: e8 cc ff ff ff call 1040 <_Z3foov>
1074: 31 c0 xor eax,eax
1076: 59 pop rcx
1077: c3 ret
</code></pre>
<h2 id="gcc"><a class="header" href="#gcc">gcc</a></h2>
<p>With <code>gcc 13.2.1</code> on the current machine, the optimizer puts <code>bar()</code> on the
<code>hot path</code> by default.</p>
<pre><code class="language-x86asm">0000000000001040 <main>:
1040: 48 83 ec 08 sub rsp,0x8
; Jump if argc == 2.
1044: 83 ff 02 cmp edi,0x2
1047: 74 0c je 1055 <main+0x15>
; bar () is on the hot path (fall-through).
1049: e8 22 01 00 00 call 1170 <_Z3barv>
104e: 31 c0 xor eax,eax
1050: 48 83 c4 08 add rsp,0x8
1054: c3 ret
; foo() is on the cold path (branch).
1055: e8 06 01 00 00 call 1160 <_Z3foov>
105a: eb f2 jmp 104e <main+0xe>
105c: 0f 1f 40 00 nop DWORD PTR [rax+0x0]
</code></pre>
<p>The following shows how to compile with profiling instrumentation and how to
optimize the final program with the collected profiling data.</p>
<p>The arguments to <code>./test</code> are chosen such that <code>2/3</code> runs call <code>foo()</code>, which
is currently on the <code>cold path</code>.</p>
<pre><code class="language-bash">gcc -o test test.cc -O3 -fprofile-generate
./test 1
./test 1
./test 2 2
gcc -o test test.cc -O3 -fprofile-use
</code></pre>
<blockquote>
<p>NOTE: Consecutive runs update the generated <code>test.gcda</code> profile data file
rather than re-write it.</p>
</blockquote>
<p>After optimizing the program with the profiling data, the <code>main()</code> function</p>
<pre><code class="language-x86asm">0000000000001040 <main.cold>:
; bar() is on the cold path (branch).
1040: e8 05 00 00 00 call 104a <_Z3barv>
1045: e9 25 00 00 00 jmp 106f <main+0xf>
0000000000001060 <main>:
1060: 51 push rcx
; Jump if argc != 2.
1061: 83 ff 02 cmp edi,0x2
1064: 0f 85 d6 ff ff ff jne 1040 <main.cold>
; for() is on the hot path (fall-through).
106a: e8 11 01 00 00 call 1180 <_Z3foov>
106f: 31 c0 xor eax,eax
1071: 5a pop rdx
1072: c3 ret
</code></pre>
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