MEMORY ARENA

Syntax

MEMORY ARENA [SUMMARY] [BACKING] [thread-id] MEMORY ARENA SHOW

Time complexity: O(N) where N is the number of heap pages inspected.

ACL categories: @read, @slow

The MEMORY ARENA command reports allocator arena statistics broken down by block size. It is a diagnostic command, useful for understanding where committed memory is going and how much of it is wasted on partially filled blocks. This command is Dragonfly-specific.

Because Dragonfly is multi-threaded and each thread allocates from its own heap, the statistics are per thread. By default the command reports the heap of thread 0; pass a thread-id to inspect a different thread.

Options

SUMMARY reports every thread in turn, followed by a machine-wide total, with the rows aggregated by block size. A thread-id cannot be combined with SUMMARY, since the summary already covers all threads.

BACKING reports the backing heap — the heap used for the server's own bookkeeping allocations — instead of the heap that holds user data.

SHOW prints a process-wide arena report to the server's standard output and replies OK. The report does not come back over the connection, and the server must have been started with the MIMALLOC_VERBOSE=1 environment variable for anything to be printed. SHOW takes no further arguments.

Return

Verbatim string reply: the arena statistics report, for every form except SHOW.

Simple string reply: OK for MEMORY ARENA SHOW.

Error reply: if thread-id is not less than the number of threads, or if an argument follows SUMMARY or SHOW.

The default report has one row per block size, with the following columns.

Column	Description
Count	Number of heap pages holding blocks of this size.
BlockSize	Size in bytes of each block in the row.
Reserved	Address space reserved for those pages.
Committed	Memory actually committed and backed by the operating system.
Used	Memory occupied by live allocations.

The SUMMARY form replaces Count with two derived columns: Wasted, the difference between committed and used memory, and Waste%, the same figure as a percentage of committed memory.

Examples

The default form reports one thread, ending with a process-level fragmentation figure:

dragonfly> MEMORY ARENA

Arena statistics from thread:0
Count BlockSize Reserved Committed Used
1 20480 512000 81920 20480
1 32 65440 4096 96
1 8 65504 4096 32
1 16 65488 4096 0
1 32768 524288 262144 262144
1 1792 62720 7168 1792
1 64 65344 4096 128
total reserved: 1360784, committed: 367616, used: 284672 fragmentation waste: 22.5627%
--- End mimalloc statistics, took 7us ---

SUMMARY sorts the same data by block size and adds the waste columns, one table per thread followed by a machine-wide table:

dragonfly> MEMORY ARENA SUMMARY

Arena statistics for thread 0:
 BlockSize   Reserved  Committed       Used     Wasted   Waste%
     32768     524288     262144     262144          0     0.00%
        64      65344       4096        128       3968    96.88%
        32      65440       4096         96       4000    97.66%
         8      65504       4096         32       4064    99.22%
        16      65488       4096          0       4096   100.00%
      1792      62720       7168       1792       5376    75.00%
     20480     512000      81920      20480      61440    75.00%
    Total:    1360784     367616     284672      82944    22.56%

Arena statistics for thread 1:
 BlockSize   Reserved  Committed       Used     Wasted   Waste%
     32768     524288     262144     262144          0     0.00%
      1792      64512       7168       3584       3584    50.00%
        64      65344       4096         64       4032    98.44%
        32      65440       4096         32       4064    99.22%
         8      65504       4096          8       4088    99.80%
        16      65488       4096          0       4096   100.00%
     20480     491520      81920      20480      61440    75.00%
    Total:    1342096     367616     286312      81304    22.12%

Arena statistics for machine:
 BlockSize   Reserved  Committed       Used     Wasted   Waste%
     32768    1048576     524288     524288          0     0.00%
        64     130688       8192        192       8000    97.66%
        32     130880       8192        128       8064    98.44%
         8     131008       8192         40       8152    99.51%
        16     130976       8192          0       8192   100.00%
      1792     127232      14336       5376       8960    62.50%
     20480    1003520     163840      40960     122880    75.00%
    Total:    2702880     735232     570984     164248    22.34%

--- End mimalloc statistics, took 38us ---

Requesting a thread that does not exist is an error, as is passing a thread id together with SUMMARY:

dragonfly> MEMORY ARENA 99
(error) ERR Thread id must be less than 2

dragonfly> MEMORY ARENA SUMMARY 0
(error) ERR syntax error

See also

MEMORY | MEMORY MALLOC-STATS | MEMORY DECOMMIT | MEMORY DEFRAGMENT