</para>
<sect1 id="jit-reason">
- <title>What is <acronym>JIT</acronym>?</title>
+ <title>What is <acronym>JIT</acronym> compilation?</title>
<para>
Just-in-time compilation (<acronym>JIT</acronym>) is the process of turning
<para>
<productname>PostgreSQL</productname> has builtin support to perform
- <acronym>JIT</acronym> using <ulink
+ <acronym>JIT</acronym>
compilation using <ulink
url="https://llvm.org/"><productname>LLVM</productname></ulink> when
<productname>PostgreSQL</productname> was built with
<literal>--with-llvm</literal> (see <xref linkend="configure-with-llvm"/>).
<title>When to <acronym>JIT</acronym>?</title>
<para>
- <acronym>JIT</acronym> is beneficial primarily for long-running CPU bound
- queries. Frequently these will be analytical queries. For short queries
- the overhead of performing <acronym>JIT</acronym> will often be higher than
- the time it can save.
+ <acronym>JIT</acronym> compilation is beneficial primarily for long-running
+ CPU bound queries. Frequently these will be analytical queries. For short
+ queries the added overhead of performing <acronym>JIT</acronym> compilation
+ will often be higher than the time it can save.
</para>
<para>
- To determine whether <acronym>JIT</acronym> is used, the total cost of a
- query (see <xref linkend="planner-stats-details"/> and <xref
+ To determine whether <acronym>JIT</acronym> compilation is used, the total
+ cost of a query (see <xref linkend="planner-stats-details"/> and <xref
linkend="runtime-config-query-constants"/>) is used.
</para>
<para>
If the planner, based on the above criterion, decided that
- <acronym>JIT</acronym> is beneficial, two further decisions are
+ <acronym>JIT</acronym> compilation is beneficial, two further decisions are
made. Firstly, if the query is more costly than the <xref
- linkend="guc-jit-optimize-above-cost"/>, GUC expensive optimizations are
+ linkend="guc-jit-optimize-above-cost"/> GUC, expensive optimizations are
used to improve the generated code. Secondly, if the query is more costly
than the <xref linkend="guc-jit-inline-above-cost"/> GUC, short functions
and operators used in the query will be inlined. Both of these operations
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
</programlisting>
As visible here, <acronym>JIT</acronym> was used, but inlining and
- optimization were not. If <xref linkend="guc-jit-optimize-above-cost"/>,
- <xref linkend="guc-jit-inline-above-cost"/> were lowered, just like <xref
+ expensive optimization were not. If <xref
+ linkend="guc-jit-optimize-above-cost"/>, <xref
+ linkend="guc-jit-inline-above-cost"/> were lowered, just like <xref
linkend="guc-jit-above-cost"/>, that would change.
</para>
</sect1>
<title>Configuration</title>
<para>
- <xref linkend="guc-jit"/> determines whether <acronym>JIT</acronym> is
- enabled or disabled.
+ <xref linkend="guc-jit"/> determines whether <acronym>JIT</acronym>
+ compilation is enabled or disabled.
</para>
<para>
That this is done at query execution time, possibly even only in cases
the relevant task is done a number of times, makes it JIT, rather than
ahead-of-time (AOT). Given the way JIT compilation is used in
-postgres, the lines between interpretation, AOT and JIT are somewhat
+PostgreSQL, the lines between interpretation, AOT and JIT are somewhat
blurry.
Note that the interpreted program turned into a native program does
not necessarily have to be a program in the classical sense. E.g. it
-is highly beneficial JIT compile tuple deforming into a native
+is highly beneficial to JIT compile tuple deforming into a native
function just handling a specific type of table, despite tuple
deforming not commonly being understood as a "program".
Why JIT?
========
-Parts of postgres are commonly bottlenecked by comparatively small
+Parts of PostgreSQL are commonly bottlenecked by comparatively small
pieces of CPU intensive code. In a number of cases that is because the
relevant code has to be very generic (e.g. handling arbitrary SQL
level expressions, over arbitrary tables, with arbitrary extensions
How to JIT
==========
-Postgres, by default, uses LLVM to perform JIT. LLVM was chosen
+PostgreSQL, by default, uses LLVM to perform JIT. LLVM was chosen
because it is developed by several large corporations and therefore
unlikely to be discontinued, because it has a license compatible with
-PostgreSQL, and because its LLVM IR can be generated from C
-using the clang compiler.
+PostgreSQL, and because its IR can be generated from C using the Clang
+compiler.
Shared Library Separation
evaluate JIT compilation that does not use LLVM, by changing out the
shared library used to provide JIT compilation.
-To achieve this code, e.g. expression evaluation, intending to perform
-JIT, calls a LLVM independent wrapper located in jit.c to do so. If
-the shared library providing JIT support can be loaded (i.e. postgres
-was compiled with LLVM support and the shared library is installed),
-the task of JIT compiling an expression gets handed of to shared
-library. This obviously requires that the function in jit.c is allowed
-to fail in case no JIT provider can be loaded.
+To achieve this, code intending to perform JIT (e.g. expression evaluation)
+calls an LLVM independent wrapper located in jit.c to do so. If the
+shared library providing JIT support can be loaded (i.e. PostgreSQL was
+compiled with LLVM support and the shared library is installed), the task
+of JIT compiling an expression gets handed off to the shared library. This
+obviously requires that the function in jit.c is allowed to fail in case
+no JIT provider can be loaded.
Which shared library is loaded is determined by the jit_provider GUC,
defaulting to "llvmjit".
Cloistering code performing JIT into a shared library unfortunately
also means that code doing JIT compilation for various parts of code
has to be located separately from the code doing so without
-JIT. E.g. the JITed version of execExprInterp.c is located in
-jit/llvm/ rather than executor/.
+JIT. E.g. the JIT version of execExprInterp.c is located in jit/llvm/
+rather than executor/.
JIT Context
Emitting individual functions separately is more expensive than
emitting several functions at once, and emitting them together can
-provide additional optimization opportunities. To facilitate that the
-LLVM provider separates function definition from emitting them in an
-executable way.
+provide additional optimization opportunities. To facilitate that, the
+LLVM provider separates defining functions from optimizing and
+emitting functions in an executable manner.
Creating functions into the current mutable module (a module
essentially is LLVM's equivalent of a translation unit in C) is done
Error Handling
--------------
-There are two aspects to error handling. Firstly, generated (LLVM IR)
+There are two aspects of error handling. Firstly, generated (LLVM IR)
and emitted functions (mmap()ed segments) need to be cleaned up both
after a successful query execution and after an error. This is done by
registering each created JITContext with the current resource owner,
LLVM itself runs out of memory. LLVM by default does *not* use any C++
exceptions. Its allocations are primarily funneled through the
standard "new" handlers, and some direct use of malloc() and
-mmap(). For the former a 'new handler' exists
-http://en.cppreference.com/w/cpp/memory/new/set_new_handler for the
-latter LLVM provides callback that get called upon failure
-(unfortunately mmap() failures are treated as fatal rather than OOM
-errors). What we've, for now, chosen to do, is to have two functions
-that LLVM using code must use:
+mmap(). For the former a 'new handler' exists:
+http://en.cppreference.com/w/cpp/memory/new/set_new_handler
+For the latter LLVM provides callbacks that get called upon failure
+(unfortunately mmap() failures are treated as fatal rather than OOM errors).
+What we've chosen to do for now is have two functions that LLVM using code
+must use:
extern void llvm_enter_fatal_on_oom(void);
extern void llvm_leave_fatal_on_oom(void);
before interacting with LLVM code.
Using a relatively small enter/leave protected section of code, rather
than setting up these handlers globally, avoids negative interactions
-with extensions that might use C++ like e.g. postgis. As LLVM code
+with extensions that might use C++ such as PostGIS. As LLVM code
generation should never execute arbitrary code, just setting these
handlers temporarily ought to suffice.
Type Synchronization
--------------------
-To able to generate code performing tasks that are done in "interpreted"
-postgres, it obviously is required that code generation knows about at
-least a few postgres types. While it is possible to inform LLVM about
+To be able to generate code that can perform tasks done by "interpreted"
+PostgreSQL, it obviously is required that code generation knows about at
+least a few PostgreSQL types. While it is possible to inform LLVM about
type definitions by recreating them manually in C code, that is failure
prone and labor intensive.
the types required for JITing. That file is translated to bitcode at
compile time, and loaded when LLVM is initialized in a backend.
-That works very well to synchronize the type definition, unfortunately
+That works very well to synchronize the type definition, but unfortunately
it does *not* synchronize offsets as the IR level representation doesn't
-know field names. Instead required offsets are maintained as defines in
-the original struct definition. E.g.
+know field names. Instead, required offsets are maintained as defines in
+the original struct definition, like so:
#define FIELDNO_TUPLETABLESLOT_NVALID 9
int tts_nvalid; /* # of valid values in tts_values */
-while that still needs to be defined, it's only required for a
+While that still needs to be defined, it's only required for a
relatively small number of fields, and it's bunched together with the
struct definition, so it's easily kept synchronized.
--------
One big advantage of JITing expressions is that it can significantly
-reduce the overhead of postgres's extensible function/operator
-mechanism, by inlining the body of called functions / operators.
+reduce the overhead of PostgreSQL's extensible function/operator
+mechanism, by inlining the body of called functions/operators.
It obviously is undesirable to maintain a second implementation of
commonly used functions, just for inlining purposes. Instead we take
-advantage of the fact that the clang compiler can emit LLVM IR.
+advantage of the fact that the Clang compiler can emit LLVM IR.
The ability to do so allows us to get the LLVM IR for all operators
(e.g. int8eq, float8pl etc), without maintaining two copies. These
Currently it is not yet possible to cache generated functions, even
though that'd be desirable from a performance point of view. The
problem is that the generated functions commonly contain pointers into
-per-execution memory. The expression evaluation functionality needs to
+per-execution memory. The expression evaluation machinery needs to
be redesigned a bit to avoid that. Basically all per-execution memory
needs to be referenced as an offset to one block of memory stored in
an ExprState, rather than absolute pointers into memory.
- jit_inline_above_cost = -1, 0-DBL_MAX - inlining is tried if query has
higher cost.
-whenever a query's total cost is above these limits, JITing is
+Whenever a query's total cost is above these limits, JITing is
performed.
Alternative costing models, e.g. by generating separate paths for
The obvious seeming approach of JITing expressions individually after
a number of execution turns out not to work too well. Primarily
because emitting many small functions individually has significant
-overhead. Secondarily because the time till JITing occurs causes
+overhead. Secondarily because the time until JITing occurs causes
relative slowdowns that eat into the gain of JIT compilation.
projects / postgresql / commitdiff