memsys

Memory Manager, Slab Allocator (MMSA)

MMSA is, simultaneously, a) Slab and SGL allocator, and b) memory manager that is responsible to optimize memory usage between different (more vs less) utilized Slabs.

Multiple MMSA instances may coexist in the system, each having its own constraints and managing its own Slabs and SGLs.

MMSA includes a "house-keeping" part to monitor system resources, adjust Slab sizes based on their respective usages, and incrementally deallocate idle Slabs. To that end, MMSA utilizes housekeep (project and runner).

Construction

A typical initialization sequence includes steps, e.g.:

1. Construct:

   mm := &memsys.MMSA{Name: ..., TimeIval: ..., MinPctFree: ..., Name: ...}

   Note that with the only exception of Name all the rest member variables (above) have their system defaults and can be omitted.

2. Initialize:

   err := mm.Init(false /* don't panic on error */)
   if err != nil {
       ...
   }

The example above shows initialization that ignores errors - in particular, insifficient minimum required memory (see the previous section).

Alternatively, MMSA can be initialized not to panic on errors:

 mm.Init(true /* panic on error */)

In addition, there are several environment variables that can be used (to circumvent the need to change the code, for instance):

AIS_MINMEM_FREE
AIS_MINMEM_PCT_TOTAL
AIS_MINMEM_PCT_FREE

Minimum Available Memory

MMSA will try to make sure that there's a certain specified amount of memory that remains available at all times. Following are the rules to set this minimum:

1. environment AIS_MINMEM_FREE takes precedence over everything else listed below;
2. if AIS_MINMEM_FREE is not defined, variables AIS_MINMEM_PCT_TOTAL and/or AIS_MINMEM_PCT_FREE define percentages to compute the minimum based on the total or the currently available memory, respectively;
3. with no environment, the minimum is computed based on the following MMSA member variables:
   • MinFree - memory that must be available at all times
   • MinPctTotal - same, via percentage of total
   • MinPctFree - ditto, as % of free at init time
   • Example:

     memsys.MMSA{MinPctTotal: 4, MinFree: cmn.GiB * 2}

4. finally, if none of the above is specified, the constant minMemFree in the source

Termination

If the memory manager is no longer needed, terminating the MMSA instance is recommended. This will free up all the slabs allocated to the memory manager instance. Halt a running or initialized MMSA instance is done by:

mm.Terminate()

Operation

Once constructed and initialized, memory-manager-and-slab-allocator (MMSA) can be exercised via its public API that includes GetSlab, on the one hand and Alloc/AllocSize on the other.

Notice the difference:

• GetSlab(fixed-bufsize) returns Slab that contains presizely fixed-bufsize sized reusable buffers
• Alloc() and AllocSize() return both a Slab and an already allocated buffer from this Slab.

Note as well that Alloc() uses default buffer size for a given MMSA, while AllocSize() accepts the specified size (as the name implies).

Once selected, each Slab can be used via its own public API that includes Alloc and Free methods. In addition, each allocated SGL internally utilizes one of the existing enumerated slabs to "grow" (that is, allocate more buffers from the slab) on demand. For details, look for "grow" in the iosgl.go.

When running, the memory manager periodically evaluates the remaining free memory resource and adjusts its slabs accordingly. The entire logic is consolidated in one work() method that can, for instance, "cleanup" (see cleanup()) an existing "idle" slab, or forcefully "reduce" (see reduce()) one if and when the amount of free memory falls below watermark.

Testing

• Run all tests in debug mode:

$ go test -v -logtostderr=true -duration 2m -tags=debug

• Run one of the named tests for 100 seconds:

$ go test -v -run=Test_Sleep -duration=100s

• Same as above with debug and deadbeef (build tags) enabled:

$ go test -v -tags=debug,deadbeef -run=Test_Sleep -duration=100s

• Run each test for 10 minutes with the permission to use up to 90% of total RAM

$ AIS_MINMEM_PCT_TOTAL=10 go test -v -run=No -duration 10m -timeout=1h -tags=debug

Global Memory Manager

In the interest of reusing a single memory manager instance across multiple packages outside the ais core package, the memsys package declares a gMem2 variable that can be accessed through the matching exported Getter. The notable runtime parameters that are used for the global memory manager are MinFreePct and TimeIval which are set to 50% and 2 minutes, respectively. Note that more specialized use cases which warrant custom memory managers with finely tuned parameters are free to create their own separate MMSA instances.

Usage:

To access the global memory manager, a single call to memsys.Init() is all that is required. Separate Init() nor Run() calls should not be made on the returned MMSA instance.