libc/stdlib/jemalloc.3

.\" $NetBSD: jemalloc.3,v 1.12 2022/12/04 01:29:32 uwe Exp $
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.\"     @(#)malloc.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: src/lib/libc/stdlib/malloc.3,v 1.73 2007/06/15 22:32:33 jasone Exp $
.\"
.Dd June 21, 2011
.Dt JEMALLOC 3
.Os
.Sh NAME
.Nm jemalloc ,
.Nm malloc.conf
.Nd the default system allocator
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Ft const char *
.Va _malloc_options ;
.Sh DESCRIPTION
The
.Nm
is a general-purpose concurrent
.Xr malloc 3
implementation specifically designed to be scalable
on modern multi-processor systems.
It is the default user space system allocator in
.Nx .
.Pp
When the first call is made to one of the memory allocation
routines such as
.Fn malloc
or
.Fn realloc ,
various flags that affect the workings of the allocator are set or reset.
These are described below.
.Pp
The
.Dq name
of the file referenced by the symbolic link named
.Pa /etc/malloc.conf ,
the value of the environment variable
.Ev MALLOC_OPTIONS ,
and the string pointed to by the global variable
.Va _malloc_options
will be interpreted, in that order, character by character as flags.
.Pp
Most flags are single letters.
Uppercase letters indicate that the behavior is set, or on,
and lowercase letters mean that the behavior is not set, or off.
The following options are available.
.Bl -tag -width "A   " -offset 3n
.It Em A
All warnings (except for the warning about unknown
flags being set) become fatal.
The process will call
.Xr abort 3
in these cases.
.It Em H
Use
.Xr madvise 2
when pages within a chunk are no longer in use, but the chunk as a whole cannot
yet be deallocated.
This is primarily of use when swapping is a real possibility, due to the high
overhead of the
.Fn madvise
system call.
.It Em J
Each byte of new memory allocated by
.Fn malloc ,
.Fn realloc
will be initialized to 0xa5.
All memory returned by
.Fn free ,
.Fn realloc
will be initialized to 0x5a.
This is intended for debugging and will impact performance negatively.
.It Em K
Increase/decrease the virtual memory chunk size by a factor of two.
The default chunk size is 1 MB.
This option can be specified multiple times.
.It Em N
Increase/decrease the number of arenas by a factor of two.
The default number of arenas is four times the number of CPUs, or one if there
is a single CPU.
This option can be specified multiple times.
.It Em P
Various statistics are printed at program exit via an
.Xr atexit 3
function.
This has the potential to cause deadlock for a multi-threaded process that exits
while one or more threads are executing in the memory allocation functions.
Therefore, this option should only be used with care; it is primarily intended
as a performance tuning aid during application development.
.It Em Q
Increase/decrease the size of the allocation quantum by a factor of two.
The default quantum is the minimum allowed by the architecture (typically 8 or
16 bytes).
This option can be specified multiple times.
.It Em S
Increase/decrease the size of the maximum size class that is a multiple of the
quantum by a factor of two.
Above this size, power-of-two spacing is used for size classes.
The default value is 512 bytes.
This option can be specified multiple times.
.It Em U
Generate
.Dq utrace
entries for
.Xr ktrace 1 ,
for all operations.
Consult the source for details on this option.
.It Em V
Attempting to allocate zero bytes will return a
.Dv NULL
pointer instead of a valid pointer.
(The default behavior is to make a minimal allocation and return a
pointer to it.)
This option is provided for System V compatibility.
This option is incompatible with the
.Em X
option.
.It Em X
Rather than return failure for any allocation function,
display a diagnostic message on
.Dv stderr
and cause the program to drop
core (using
.Xr abort 3 ) .
This option should be set at compile time by including the following in
the source code:
.Bd -literal -offset indent
_malloc_options = "X";
.Ed
.It Em Z
Each byte of new memory allocated by
.Fn malloc ,
.Fn realloc
will be initialized to 0.
Note that this initialization only happens once for each byte, so
.Fn realloc
does not zero memory that was previously allocated.
This is intended for debugging and will impact performance negatively.
.El
.Pp
Extra care should be taken when enabling
any of the options in production environments.
The
.Em A ,
.Em J ,
and
.Em Z
options are intended for testing and debugging.
An application which changes its behavior when these options are used
is flawed.
.Sh IMPLEMENTATION NOTES
The
.Nm
allocator uses multiple arenas in order to reduce lock
contention for threaded programs on multi-processor systems.
This works well with regard to threading scalability, but incurs some costs.
There is a small fixed per-arena overhead, and additionally, arenas manage
memory completely independently of each other, which means a small fixed
increase in overall memory fragmentation.
These overheads are not generally an issue,
given the number of arenas normally used.
Note that using substantially more arenas than the default is not likely to
improve performance, mainly due to reduced cache performance.
However, it may make sense to reduce the number of arenas if an application
does not make much use of the allocation functions.
.Pp
Memory is conceptually broken into equal-sized chunks,
where the chunk size is a power of two that is greater than the page size.
Chunks are always aligned to multiples of the chunk size.
This alignment makes it possible to find
metadata for user objects very quickly.
.Pp
User objects are broken into three categories according to size:
.Bl -enum -offset 3n
.It
Small objects are smaller than one page.
.It
Large objects are smaller than the chunk size.
.It
Huge objects are a multiple of the chunk size.
.El
.Pp
Small and large objects are managed by arenas; huge objects are managed
separately in a single data structure that is shared by all threads.
Huge objects are used by applications infrequently enough that this single
data structure is not a scalability issue.
.Pp
Each chunk that is managed by an arena tracks its contents in a page map as
runs of contiguous pages (unused, backing a set of small objects, or backing
one large object).
The combination of chunk alignment and chunk page maps makes it possible to
determine all metadata regarding small and large allocations in constant time.
.Pp
Small objects are managed in groups by page runs.
Each run maintains a bitmap that tracks which regions are in use.
Allocation requests can be grouped as follows.
.Bl -bullet -offset 3n
.It
Allocation requests that are no more than half the quantum (see the
.Em Q
option) are rounded up to the nearest power of two (typically 2, 4, or 8).
.It
Allocation requests that are more than half the quantum, but no more than the
maximum quantum-multiple size class (see the
.Em S
option) are rounded up to the nearest multiple of the quantum.
.It
Allocation requests that are larger than the maximum quantum-multiple size
class, but no larger than one half of a page, are rounded up to the nearest
power of two.
.It
Allocation requests that are larger than half of a page, but small enough to
fit in an arena-managed chunk (see the
.Em K
option), are rounded up to the nearest run size.
.It
Allocation requests that are too large to fit in an arena-managed chunk are
rounded up to the nearest multiple of the chunk size.
.El
.Pp
Allocations are packed tightly together, which can be an issue for
multi-threaded applications.
If you need to assure that allocations do not suffer from cache line sharing,
round your allocation requests up to the nearest multiple of the cache line
size.
.Sh DEBUGGING
The first thing to do is to set the
.Em A
option.
This option forces a coredump (if possible) at the first sign of trouble,
rather than the normal policy of trying to continue if at all possible.
.Pp
It is probably also a good idea to recompile the program with suitable
options and symbols for debugger support.
.Pp
If the program starts to give unusual results, coredump or generally behave
differently without emitting any of the messages mentioned in the next
section, it is likely because it depends on the storage being filled with
zero bytes.
Try running it with the
.Em Z
option set;
if that improves the situation, this diagnosis has been confirmed.
If the program still misbehaves,
the likely problem is accessing memory outside the allocated area.
.Pp
Alternatively, if the symptoms are not easy to reproduce, setting the
.Em J
option may help provoke the problem.
In truly difficult cases, the
.Em U
option, if supported by the kernel, can provide a detailed trace of
all calls made to these functions.
.Pp
Unfortunately,
.Nm
does not provide much detail about the problems it detects;
the performance impact for storing such information would be prohibitive.
There are a number of allocator implementations available on the Internet
which focus on detecting and pinpointing problems by trading performance for
extra sanity checks and detailed diagnostics.
.Sh ENVIRONMENT
The following environment variables affect the execution of the allocation
functions:
.Bl -tag -width ".Ev MALLOC_OPTIONS"
.It Ev MALLOC_OPTIONS
If the environment variable
.Ev MALLOC_OPTIONS
is set, the characters it contains will be interpreted as flags to the
allocation functions.
.El
.Sh EXAMPLES
To dump core whenever a problem occurs:
.Bd -literal -offset indent
ln -s 'A' /etc/malloc.conf
.Ed
.Pp
To specify in the source that a program does no return value checking
on calls to these functions:
.Bd -literal -offset indent
_malloc_options = "X";
.Ed
.Sh RETURN VALUES
If any of the memory allocation/deallocation functions detect an error or
warning condition, a message will be printed to file descriptor
.Dv STDERR_FILENO .
Errors will result in the process dumping core.
If the
.Em A
option is set, all warnings are treated as errors.
.Pp
.\"
.\" XXX: The _malloc_message should be documented
.\"	 better in order to be worth mentioning.
.\"
The
.Va _malloc_message
variable allows the programmer to override the function which emits
the text strings forming the errors and warnings if for some reason
the
.Dv stderr
file descriptor is not suitable for this.
Please note that doing anything which tries to allocate memory in
this function is likely to result in a crash or deadlock.
.Pp
All messages are prefixed by
.Dq Ao Ar progname Ac Ns Li \&: Pq malloc .
.Sh SEE ALSO
.Xr emalloc 3 ,
.Xr malloc 3 ,
.Xr memory 3 ,
.Xr memoryallocators 9
.\"
.\" XXX: Add more references that could be worth reading.
.\"
.Rs
.%A Jason Evans
.%T "A Scalable Concurrent malloc(3) Implementation for FreeBSD"
.%D April 16, 2006
.%O BSDCan 2006
.%U http://people.freebsd.org/~jasone/jemalloc/bsdcan2006/jemalloc.pdf
.Re
.Rs
.%A Poul-Henning Kamp
.%T "Malloc(3) revisited"
.%I USENIX Association
.%B Proceedings of the FREENIX Track: 1998 USENIX Annual Technical Conference
.%D June 15-19, 1998
.%U http://www.usenix.org/publications/library/proceedings/usenix98/freenix/kamp.pdf
.Re
.Rs
.%A Paul R. Wilson
.%A Mark S. Johnstone
.%A Michael Neely
.%A David Boles
.%T "Dynamic Storage Allocation: A Survey and Critical Review"
.%D 1995
.%I University of Texas at Austin
.%U ftp://ftp.cs.utexas.edu/pub/garbage/allocsrv.ps
.Re
.Sh HISTORY
The
.Nm
allocator became the default system allocator first in
.Fx 7.0
and then in
.Nx 5.0 .
In both systems it replaced the older so-called
.Dq phkmalloc
implementation.
.Sh AUTHORS
.An Jason Evans Aq Mt jasone@canonware.com