Well, it is hard to enumerate. The basic idea is to find, for each data structure, the best trade-off between efficient operations and space occupation. The simplest example is to decide between using lists or arrays. Arrays use less space (unless you use hash-consing or you share the end of the lists), but if you don't know the final size you need, you will finish either pre-allocating longer arrays than you need, or re-allocating arrays too often, or switching between lists and arrays... In the end, it really depends on what an application does. 

There are also other tricks such as changing the tag of a block to avoid scanning, but I wouldn't advise to use it unless you really know what you are doing...

--Fabrice

On Thu, Jul 24, 2014 at 5:39 PM, Malcolm Matalka <mmatalka@gmail.com> wrote:

Cool, what sort of tricks can you do to reduce the number of blocks?

Den 24 jul 2014 17:36 skrev "Fabrice Le Fessant" <Fabrice.Le_fessant@inria.fr>:

Note that the cost of the GC does not automatically depends on the size of RAM. In many networking servers, memory is filled with strings, caching files on disk or content to be sent on the network. Such cases make OCaml GC happy, since it does not have to manipulate many objects, and it won't scan strings for pointers within them. There are also other tricks to improve the GC behavior: you might want to change the data representation to decrease the number of blocks in the heap, I used to do it a lot when doing computations on millions of entries that would not otherwise stay in memory.

--Fabrice

On Thu, Jul 24, 2014 at 8:58 AM, Nicolas Boulay <nicolas@boulay.name> wrote:

What about server that use ~60GB of RAM ? Todays server are sold with 32 to 256 GB of RAM and lot of cpu core.

Maybe in such extreme cases, offloading the major collection of the GC could reduce latency a lot ?

2014-07-24 2:05 GMT+02:00 John F. Carr <jfc@mit.edu>:

Most programs spend a minority of their time in garbage collection.
Even if the new GC thread did not slow down the main program,
possible speedup would be less than 2x, probably well under 50%.

For technical reasons, offloading major collections in OCaml is easier
than offloading minor collections, so the potential benefit is less.

 > extremely clueless question warning, both generally technically but
 > also vis-a-vie ocaml specifically:
 >
 > so even if ocaml can't so easily be made to support multiple threads
 > of ocaml code, could the gc be moved off to another thread? so that it
 > could run on another core. would that be of any benefit?

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--
Fabrice LE FESSANT
Chercheur en Informatique
INRIA Paris Rocquencourt -- OCamlPro
Programming Languages and Distributed Systems

--
Fabrice LE FESSANT
Chercheur en Informatique
INRIA Paris Rocquencourt -- OCamlPro
Programming Languages and Distributed Systems