One awesome GC feature we will use in Mir!

[9il]

On Tuesday, 18 September 2018 at 15:55:38 UTC, Nordlöw wrote:
> On Tuesday, 18 September 2018 at 14:23:44 UTC, 9il wrote:
>> I just remember that D's GC has NO_SCAN [1] attribute!
>>
>> This will be added by default when for Mir allocations if type 
>> representation tuple has not references. For example, are 
>> Slice!(double*, 2) should never be scanned by GC, but it will 
>> be in GC heap until something refers it.
>>
>> Let me know if you have ideas how to further improve memory 
>> management and required API in Mir and Lubeck.
>>
>> [1] 
>> https://dlang.org/phobos/core_memory.html#.GC.BlkAttr.NO_SCAN
>>
>> Best,
>> Ilya
>
>  Can you elaborate on why this is the case?

Mir users work with time-series, matrixes, tensors. A lot of 
numeric and scientific data. Almost all structures are plain. 
mir.series is used instead of associative arrays. Associative 
arrays are used only to define data set, and then AA converted to 
Series of immutable (represented as two arrays). In practice 99% 
of data are plain arrays, and ~80% of this arrays are arrays 
composed of doubles, ints, or POD structs. Such types does not 
contains references to other GC allocated memory. So, we can 
reduce GC latency 5 times for production code.

If a user allocates new double[], GC will scan whole array 
memory, because it is assumed that user may reuse this memory for 
types that have references.

So, the main idea, is that if one allocates a Matrix of doubles, 
then just turn off scanning of its internal data. Casting from 
array of doubles to say strings is not @safe, so we have the 
language instrument to prevent memory leaks for user code.

As side effect this will reduce false pointers too.