4 ms·

I see cache related optimizations being discussed most often in the context of C or C++. Being inexperienced in this area, I wonder how would does it affect gar

by phserr 12y ago

I see cache related optimizations being discussed most often in the context of C or C++. Being inexperienced in this area, I wonder how would does it affect garbage collected languages/systems. I understand that that are different approaches to GC, but can someone recommend some discussion to read on this topic? (cache optimization either in GC development itself or in development with GC languages)

voidlogic 12y ago
Are you concerned about garbage collectors that do relocation/compaction or what? Otherwise garbage collection doesn't really effect much of what is discussed here (except of course if you are preempted by the GC running and it empties the cache of useful things (to you), but this is rare). Maybe when you talk about GC you mean the heap allocator (which of course is related but different)?
- phserr 12y ago
  What I meant was just how having a less direct control over memory (like you do with c/c++) affects (edit: in a practical way) the efficiency of cache usage. Having less control means being unable to pack things together as much as one would want, but I'm sure sophisticated GC algorithms (relocating/compacting/generational/incremental) directly or indirectly deal with that somehow. As I said, I'm inexperienced with GC and my understanding of them is probably a bit naive.
  - sliverstorm 12y ago
    From what I've read, the real worry with (for example) C++ is not whether you can control packing or memory ordering. The real worry is when you create data structures that are fundamentally cache-unfriendly. The compiler won't reachitect an object into other objects to make it more memory friendly. Simple example from the slides on optimizing for the PlayStation 3; if your object is large and full of different data, you miss a lot and burn lots of cache space every time you access it because it is so large. If your object is small and just a collection of pointers, you burn little cache space when you load it, and data can be arranged more appropriately. http://research.scee.net/files/presentations/gcapaustralia09/Pitfalls_of_Object_Oriented_Programming_GCAP_09.pdf http://research.scee.net/files/presentations/gcapaustralia09...
  - voidlogic 12y ago
    Just to you are aware, you are presenting a false dichotomy. There are plenty of languages that are both garbage collected and give you quite a bit of control about memory layout (D, Go, Rust, etc). For example, in Go if I wanted have a slice of Foobar ([]Foobar) but wanted to have them allocated close to one another I could write: const neededFoobars = 100 backingSlice := make([]Foobar, neededFoobars) seqfoobarPtrs := make([]*Foobar, neededFoobars) for i := 0; i < neededFoobars; i++ { seqfoobarPtrs[i] = &backingSlice[i] }
    - dbaupp 12y ago
      (FWIW, Rust is not garbage collected.)
      - voidlogic 12y ago
        Rust has optional garbage collection
        FreeFull 12y ago
        There is a library type that does reference counting. It's not built into the language, beyond happening to be in the standard libraries.
        dbaupp 12y ago
        It doesn't. There is a type called Gc that may be garbage collected in future, but at the moment it is just a bad reference counted pointer (the Rc type is better if you want reference counting). And it's not even guaranteed that a GC will ever be implemented, Rust provides other abstractions so GC would be rarely used even if it was implemented.
Arelius 12y ago
The same principals apply, it just becomes more difficult to understand what your language may be doing for you in this regard. Compound types in most languages will store their members contiguously, at least in cases where easy to do so. And arrays are generally contiguous in addition. The problems often crop up in not being able to have compound value-types and are instead often included by reference. Some languages, including C# IIRC, allow you to define specific types to be treated by value (structs in C#) which can often help. It may also be better to have a few arrays with primitive types in them, rather than a single array of compound types, for instance.

by phserr 12y ago

voidlogic 12y ago

Are you concerned about garbage collectors that do relocation/compaction or what? Otherwise garbage collection doesn't really effect much of what is discussed here (except of course if you are preempted by the GC running and it empties the cache of useful things (to you), but this is rare). Maybe when you talk about GC you mean the heap allocator (which of course is related but different)?

phserr 12y ago
What I meant was just how having a less direct control over memory (like you do with c/c++) affects (edit: in a practical way) the efficiency of cache usage. Having less control means being unable to pack things together as much as one would want, but I'm sure sophisticated GC algorithms (relocating/compacting/generational/incremental) directly or indirectly deal with that somehow. As I said, I'm inexperienced with GC and my understanding of them is probably a bit naive.
- sliverstorm 12y ago
  From what I've read, the real worry with (for example) C++ is not whether you can control packing or memory ordering. The real worry is when you create data structures that are fundamentally cache-unfriendly. The compiler won't reachitect an object into other objects to make it more memory friendly. Simple example from the slides on optimizing for the PlayStation 3; if your object is large and full of different data, you miss a lot and burn lots of cache space every time you access it because it is so large. If your object is small and just a collection of pointers, you burn little cache space when you load it, and data can be arranged more appropriately. http://research.scee.net/files/presentations/gcapaustralia09/Pitfalls_of_Object_Oriented_Programming_GCAP_09.pdf http://research.scee.net/files/presentations/gcapaustralia09...
- voidlogic 12y ago
  Just to you are aware, you are presenting a false dichotomy. There are plenty of languages that are both garbage collected and give you quite a bit of control about memory layout (D, Go, Rust, etc). For example, in Go if I wanted have a slice of Foobar ([]Foobar) but wanted to have them allocated close to one another I could write: const neededFoobars = 100 backingSlice := make([]Foobar, neededFoobars) seqfoobarPtrs := make([]*Foobar, neededFoobars) for i := 0; i < neededFoobars; i++ { seqfoobarPtrs[i] = &backingSlice[i] }
  - dbaupp 12y ago
    (FWIW, Rust is not garbage collected.)
    - voidlogic 12y ago
      Rust has optional garbage collection
      - FreeFull 12y ago
        There is a library type that does reference counting. It's not built into the language, beyond happening to be in the standard libraries.
      - dbaupp 12y ago
        It doesn't. There is a type called Gc that may be garbage collected in future, but at the moment it is just a bad reference counted pointer (the Rc type is better if you want reference counting). And it's not even guaranteed that a GC will ever be implemented, Rust provides other abstractions so GC would be rarely used even if it was implemented.

Arelius 12y ago

The same principals apply, it just becomes more difficult to understand what your language may be doing for you in this regard. Compound types in most languages will store their members contiguously, at least in cases where easy to do so. And arrays are generally contiguous in addition. The problems often crop up in not being able to have compound value-types and are instead often included by reference. Some languages, including C# IIRC, allow you to define specific types to be treated by value (structs in C#) which can often help. It may also be better to have a few arrays with primitive types in them, rather than a single array of compound types, for instance.