---
layout: manual
Content-Style: 'text/css'
title: _MP(PMEMOBJ_CTL_GET, 3)
collection: libpmemobj
header: PMDK
date: pmemobj API version 2.3
...

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[comment]: <> (pmemobj_ctl_get.3 -- man page for libpmemobj CTL)

[NAME](#name)<br />
[SYNOPSIS](#synopsis)<br />
[DESCRIPTION](#description)<br />
[CTL NAMESPACE](#ctl-namespace)<br />
[CTL EXTERNAL CONFIGURATION](#ctl-external-configuration)<br />
[SEE ALSO](#see-also)<br />


# NAME #

_UW(pmemobj_ctl_get),
_UW(pmemobj_ctl_set),
_UW(pmemobj_ctl_exec)
- Query and modify libpmemobj internal behavior (EXPERIMENTAL)


# SYNOPSIS #

```c
#include <libpmemobj.h>

_UWFUNCR2(int, pmemobj_ctl_get, PMEMobjpool *pop, *name, void *arg,
	=q= (EXPERIMENTAL)=e=)
_UWFUNCR2(int, pmemobj_ctl_set, PMEMobjpool *pop, *name, void *arg,
	=q= (EXPERIMENTAL)=e=)
_UWFUNCR2(int, pmemobj_ctl_exec, PMEMobjpool *pop, *name, void *arg,
	=q= (EXPERIMENTAL)=e=)
```

_UNICODE()


# DESCRIPTION #

The _UW(pmemobj_ctl_get), _UW(pmemobj_ctl_set) and _UW(pmemobj_ctl_exec)
functions provide a uniform interface for querying and modifying the internal
behavior of **libpmemobj**(7) through the control (CTL) namespace.

The *name* argument specifies an entry point as defined in the CTL namespace
specification. The entry point description specifies whether the extra *arg* is
required. Those two parameters together create a CTL query. The functions and
the entry points are thread-safe unless
indicated otherwise below. If there are special conditions for calling an entry
point, they are explicitly stated in its description. The functions propagate
the return value of the entry point. If either *name* or *arg* is invalid, -1
is returned.

If the provided ctl query is valid, the CTL functions will always return 0
on success and -1 on failure, unless otherwise specified in the entry point
description.

See more in **pmem_ctl**(5) man page.


# CTL NAMESPACE #

prefault.at_create | rw | global | int | int | - | boolean

If set, every page of the pool will be touched and written to when the pool
is created, in order to trigger page allocation and minimize the performance
impact of pagefaults. Affects only the _UW(pmemobj_create) function.

prefault.at_open | rw | global | int | int | - | boolean

If set, every page of the pool will be touched and written to when the pool
is opened, in order to trigger page allocation and minimize the performance
impact of pagefaults. Affects only the _UW(pmemobj_open) function.

sds.at_create | rw | global | int | int | - | boolean

If set, force-enables or force-disables SDS feature during pool creation.
Affects only the _UW(pmemobj_create) function. See **pmempool_feature_query**(3)
for informations about SDS (SHUTDOWN_STATE) feature.

tx.debug.skip_expensive_checks | rw | - | int | int | - | boolean

Turns off some expensive checks performed by the transaction module in "debug"
builds. Ignored in "release" builds.

tx.cache.size | rw | - | long long | long long | - | integer

Size in bytes of the transaction snapshot cache. In a larger cache the
frequency of persistent allocations is lower, but with higher fixed cost.

This should be set to roughly the sum of sizes of the snapshotted regions in
an average transaction in the pool.

This entry point is not thread safe and should not be modified if there are any
transactions currently running.

This value must be a in a range between 0 and **PMEMOBJ_MAX_ALLOC_SIZE**,
otherwise this entry point will fail.

tx.cache.threshold | rw | - | long long | long long | - | integer

This entry point is deprecated.
All snapshots, regardless of the size, use the transactional cache.

tx.post_commit.queue_depth | rw | - | int | int | - | integer

This entry point is deprecated.

tx.post_commit.worker | r- | - | void * | - | - | -

This entry point is deprecated.

tx.post_commit.stop | r- | - | void * | - | - | -

This entry point is deprecated.

heap.alloc_class.[class_id].desc | rw | - | `struct pobj_alloc_class_desc` |
`struct pobj_alloc_class_desc` | - | integer, integer, integer, string

Describes an allocation class. Allows one to create or view the internal
data structures of the allocator.

Creating custom allocation classes can be beneficial for both raw allocation
throughput, scalability and, most importantly, fragmentation. By carefully
constructing allocation classes that match the application workload,
one can entirely eliminate external and internal fragmentation. For example,
it is possible to easily construct a slab-like allocation mechanism for any
data structure.

The `[class_id]` is an index field. Only values between 0-254 are valid.
If setting an allocation class, but the `class_id` is already taken, the
function will return -1.
The values between 0-127 are reserved for the default allocation classes of the
library and can be used only for reading.

The recommended method for retrieving information about all allocation classes
is to call this entry point for all class ids between 0 and 254 and discard
those results for which the function returns an error.

This entry point takes a complex argument.

```
struct pobj_alloc_class_desc {
	size_t unit_size;
	size_t alignment;
	unsigned units_per_block;
	enum pobj_header_type header_type;
	unsigned class_id;
};
```

The first field, `unit_size`, is an 8-byte unsigned integer that defines the
allocation class size. While theoretically limited only by
**PMEMOBJ_MAX_ALLOC_SIZE**, for most workloads this value should be between
8 bytes and 2 megabytes.

The `alignment` field specifies the user data alignment of objects allocated
using the class. If set, must be a power of two and an even divisor of unit
size. Alignment is limited to maximum of 2 megabytes.
All objects have default alignment of 64 bytes, but the user data alignment
is affected by the size of the chosen header.

The `units_per_block` field defines how many units a single block of memory
contains. This value will be adjusted to match the internal size of the
block (256 kilobytes or a multiple thereof). For example, given a class with
a `unit_size` of 512 bytes and a `units_per_block` of 1000, a single block of
memory for that class will have 512 kilobytes.
This is relevant because the bigger the block size, the less frequently blocks
need to be fetched, resulting in lower contention on global heap state.
If the CTL call is being done at runtime, the `units_per_block` variable of the
provided alloc class structure is modified to match the actual value.

The `header_type` field defines the header of objects from the allocation class.
There are three types:

 - **POBJ_HEADER_LEGACY**, string value: `legacy`. Used for allocation classes
	prior to version 1.3 of the library. Not recommended for use.
	Incurs a 64 byte metadata overhead for every object.
	Fully supports all features.
 - **POBJ_HEADER_COMPACT**, string value: `compact`. Used as default for all
	predefined allocation classes.
	Incurs a 16 byte metadata overhead for every object.
	Fully supports all features.
 - **POBJ_HEADER_NONE**, string value: `none`. Header type that
	incurs no metadata overhead beyond a single bitmap entry. Can be used
	for very small allocation classes or when objects must be adjacent to
	each other.
	This header type does not support type numbers (type number is always
	0) or allocations that span more than one unit.

The `class_id` field is an optional, runtime-only variable that allows the
user to retrieve the identifier of the class. This will be equivalent to the
provided `[class_id]`. This field cannot be set from a config file.

The allocation classes are a runtime state of the library and must be created
after every open. It is highly recommended to use the configuration file to
store the classes.

This structure is declared in the `libpmemobj/ctl.h` header file. Please refer
to this file for an in-depth explanation of the allocation classes and relevant
algorithms.

Allocation classes constructed in this way can be leveraged by explicitly
specifying the class using **POBJ_CLASS_ID(id)** flag in **pmemobj_tx_xalloc**()/**pmemobj_xalloc**()
functions.

Example of a valid alloc class query string:
```
heap.alloc_class.128.desc=500,0,1000,compact
```
This query, if executed, will create an allocation class with an id of 128 that
has a unit size of 500 bytes, has at least 1000 units per block and uses
a compact header.

For reading, function returns 0 if successful, if the allocation class does
not exist it sets the errno to **ENOENT** and returns -1;

This entry point can fail if any of the parameters of the allocation class
is invalid or if exactly the same class already exists.

heap.alloc_class.new.desc | -w | - | - | `struct pobj_alloc_class_desc` | - | integer, integer, integer, string

Same as `heap.alloc_class.[class_id].desc`, but instead of requiring the user
to provide the class_id, it automatically creates the allocation class with the
first available identifier.

This should be used when it's impossible to guarantee unique allocation class
naming in the application (e.g. when writing a library that uses libpmemobj).

The required class identifier will be stored in the `class_id` field of the
`struct pobj_alloc_class_desc`.

stats.enabled | rw | - | int | int | - | boolean

Enables or disables runtime collection of statistics. Statistics are not
recalculated after enabling; any operations that occur between disabling and
re-enabling will not be reflected in subsequent values.

Statistics are disabled by default. Enabling them may have non-trivial
performance impact.

stats.heap.curr_allocated | r- | - | int | - | - | -

Reads the number of bytes currently allocated in the heap. If statistics were
disabled at any time in the lifetime of the heap, this value may be
inaccurate.

heap.size.granularity | rw- | - | uint64_t | uint64_t | - | long long

Reads or modifies the granularity with which the heap grows when OOM.
Valid only if the poolset has been defined with directories.

A granularity of 0 specifies that the pool will not grow automatically.

This entry point can fail if the granularity value is non-zero and smaller
than *PMEMOBJ_MIN_PART*.

heap.size.extend | --x | - | - | - | uint64_t | -

Extends the heap by the given size. Must be larger than *PMEMOBJ_MIN_PART*.

This entry point can fail if the pool does not support extend functionality or
if there's not enough space left on the device.

debug.heap.alloc_pattern | rw | - | int | int | - | -

Single byte pattern that is used to fill new uninitialized memory allocation.
If the value is negative, no pattern is written. This is intended for
debugging, and is disabled by default.

# CTL EXTERNAL CONFIGURATION #

In addition to direct function call, each write entry point can also be set
using two alternative methods.

The first method is to load a configuration directly from the **PMEMOBJ_CONF**
environment variable.

The second method of loading an external configuration is to set the
**PMEMOBJ_CONF_FILE** environment variable to point to a file that contains
a sequence of ctl queries.

See more in **pmem_ctl**(5) man page.


# SEE ALSO #

**libpmemobj**(7), **pmem_ctl**(5) and **<http://pmem.io>**
