Documentation Source Text

<li> Add the -withoutnulls option to the [TCL interface eval method].
<li> Enhance the [sqlite3_analyzer.exe] utility program so that it shows
     the number of bytes of metadata on btree pages.
<li> The [SQLITE_DBCONFIG_ENABLE_QPSG] run-time option and the
     [SQLITE_ENABLE_QPSG] compile-time option enable the
     [query planner stability guarantee].  See also ticket
     [https://www.sqlite.org/src/info/892fc34f173e99d8|892fc34f173e99d8]
<li> Miscellaneous optimizations result in a 2% reduction in [CPU cycles used].
<p><b>Bug Fixes:</b>
<li> Fix the query planner so that it knows not to use [automatic indexes]
     on the right table of LEFT JOIN if the WHERE clause uses the [IS operator].
     Fix for [https://sqlite.org/src/info/ce68383bf6aba|ce68383bf6aba].
<li> Ensure that the query planner knows that any column of a 
     [flattening optimization|flattened] LEFT JOIN can be NULL even 
     if that column is labeled with "NOT NULL". Fix for ticket 

<title>Measuring and Reducing CPU Usage in SQLite</title>
<tcl>hd_keywords {CPU cycles used} {CPU performance measurement}</tcl>

<table_of_contents>

<h1>Overview</h1>

<p>The graph below shows the number of CPU cycles used by SQLite on a
standard workload, for all versions of SQLite going back about 5.5 years.
The number of CPU cycles used by SQLite has been
halved in just the past three years.

<p>
This article describes how the SQLite developers measure CPU usage,
what those measurements actually mean, and the techniques used by
SQLite developers on their continuing quest to further reduce the
CPU usage of the SQLite library.
</p>

<center>
<hr>
<div class="imgcontainer">
<img src="./images/cpu-usage.jpg"></img></div><br>
Measured using cachegrind on Ubuntu 16.04 on x64 with gcc 5.4.0 and -Os.<br>
Values normalized so that version 3.20.0 is 100%.
<hr>
</center>

<h1>Measuring Performance</h1>

<p>In brief, the CPU performance of SQLite is measured as follows:

-O2 option creates so much code movement that it is difficult to associate
specific CPU instructions to C source code lines.

<h2>Workload</h2>

<p>
The "typical" workload is generated by the
[https://sqlite.org/src/file/test/speedtest1.c|speedtest1.c]
program in the canonical SQLite source tree.  This program strives to
exercise the SQLite library in a way that is typical of real-world
applications.  Of course, every application is different, and so
no test program can exactly mirror the behavior of all applications.

<p>
The speedtest1.c program is updated from time to time as the SQLite
developers' understanding of what constitutes "typical" usage evolves.

<p>
The 
[https://sqlite.org/src/file/tool/speed-check.sh|speed-check.sh] shell
script, also in the canonical source tree, is used to run the speedtest1.c
program.  To replicate the performance measurements, collect the following
files into a single directory:
<ul>
<li> the "speed-check.sh" script,
<li> the "speedtest1.c" test program, and
<li> the [amalgamation|SQLite amalgamation] source files "sqlite3.c" and
     "sqlite3.h"
</ul>
<p>
Then run "sh speed-check.sh trunk".


<h2>Performance Measurement</h2>

<p>
[http://valgrind.org/docs/manual/cg-manual.html|Cachegrind] is used to
measure performance because it gives answers that are repeatable to 
7 or more significant digits.  In comparison, actual (wall-clock)

real-world timings.  But hundreds or thousands of microoptimizations
add up, resulting in measurable real-world performance gains.

<h1>Performance Measurement Workflow</h1>

<p>
As SQLite developers edit the SQLite source code, they run the
[https://sqlite.org/src/file/tool/speed-check.sh | speed-check.sh]
shell script to track the performance impact of changes.  This
script compiles the speedtest1.c program, runs it under cachegrind,
processes the cachegrind output using the
[https://sqlite.org/src/file/tool/cg_anno.tcl | cg_anno.tcl] TCL
script, then saves the results in a series of text files.
Typical output from the speed-check.sh script looks like this:

<blockquote><pre>
==16429== I   refs:      <b><font color="red">1,291,005,499</font></b>
==16429== I1  misses:       24,688,182
==16429== LLi misses:            5,027

<p>The important parts of the output (the parts that the developers pay
the most attention to) are shown in red.
Basically, the developers want to know the size of the compiled SQLite
library and how many CPU cycles were needed to run the performance test.

<p>The output from the 
[https://sqlite.org/src/file/tool/cg_anno.tcl | cg_anno.tcl] script
shows the number of CPU cycles spent on each line of code.
The report is approximately 80,000 lines long.  The following is a brief
snippet taken from the middle of the report to show what it looks like:

<hr>
<blockquote><pre>
         .  SQLITE_PRIVATE int sqlite3BtreeNext(BtCursor *pCur, int *pRes){