The .m namespace

Modules

From kdb+ 5.0 onwards, the .m namespace was repurposed to hold the local namespaces of modules. Objects in the new .m namespace should not be accessed directly but only by using the interface provided by the respective module.

Existing code that uses .m for memory domains will not work in kdb+ 5.0+ and should be updated to remove this usage to avoid unexpected issues.

Memory backed files

V4.0 only (since 2020.03.17). For V5.0 and above see modules.

Memory can be backed by a filesystem, allowing use of DAX-enabled filesystems (e.g. AppDirect) as a non-persistent memory extension for kdb+.

Optane Memory and kdb+

Command-line option -m path directs kdb+ to use the filesystem path specified as a separate memory domain. This splits every thread’s heap into two:

domain	description
0	regular anonymous memory, active and used for all allocs by default
1	filesystem-backed memory

The .m namespace is reserved for objects in memory domain 1, however names from other namespaces can reference them too, e.g. a:.m.a:1 2 3

\d .m changes current memory domain to 1, causing it to be used by all further allocs. \d .anyotherns sets it back to 0.

.m.x:x ensures the entirety of .m.x is in memory domain 1, performing a deep copy of x as needed. (Objects of types 100h-103h, 112h are not copied and remain in memory domain 0.)

Lambdas defined in .m set current memory domain to 1 during execution. This will nest, since other lambdas don’t change memory domains:

q)\d .myns
q)g:{til x}
q)\d .m
q)w:{system"w"};f:{.myns.g x}
q)\d .
q)x:.m.f 1000000;.m.w` / x allocated in domain 1

Internal function -120!x returns x’s memory domain, currently 0 or 1.

q)-120!'(1 2 3;.m.x:1 2 3)
0 1

System command \w returns memory info for the current memory domain only.

q)value each ("\\d .m";"\\w";"\\d .";"\\w")
::
353968 67108864 67108864 0 0 8589934592
::
354032 67108864 67108864 0 0 8589934592

Command-line option -w limit (M1/m2) is no longer thread-local, but memory domain-local. Command-line option -w, and system command \w set limit for memory domain 0.