Re: GC and userdata objects

[Dibyendu Majumdar <mobile@...>]

On 12 April 2018 at 06:31, Dibyendu Majumdar <mobile@majumdar.org.uk> wrote:
> On 11 April 2018 at 22:45, Roberto Ierusalimschy <roberto@inf.puc-rio.br> wrote:
>> So, keeping some huge userdata is not particularly relevant to the
>> GC. What is relevant is the allocation of a huge userdata. You can
>> communicate this to the GC by calling collectgarbage("step", x) with 'x'
>> proportional to the size of the userdata.
>>
>
> Hi. I have tried using GC step before but did not quite succeed in
> resolving similar issues but that could have been due to my lack of
> understanding. Is there an example of how it should be used?
>
> The scenario is this - I am creating a AST generator for Lua; the AST
> is captured in a userdata. The AST is generated every time some code
> is parsed - so you can imagine that this can be quite frequent. Say in
> a test program, many code chunks are parsed resulting in several
> userdata objects of different sizes being created. I noticed that the
> GC does not collect the objects fast enough. In this scenario, how do
> you suggest I should use the GC step parameter? Keep setting it to
> different values based on userdata sizes? Or keep track of total
> userdata memory and set this accordingly? Obviously the rate of
> allocations varies over time, and sizes vary as well.
>

Just to clarify - a single fulluserdata object is created per AST -
but internally this holds its own set of data structures which are
allocated using an internal Arena based allocator. So the userdata
object is small but can point to large chunk of memory.

Also the memory gets freed when the __gc metamethod is called.

Regards
Dibyendu