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0 pointscoder5434y ago0 comments

I edited my comment before you posted yours. Making heap allocations super complicated just to avoid the stack is a confusing idea.

Generational GCs surely do not do a single bump allocate for all local variables. How could the GC possibly know where each object starts and ends if it did it as a single allocation? Instead, it treats them all as individual allocations within an allocation buffer, which means bumping for each one separately. Yes, they will then get copied out if they survive long enough, but that’s not the same thing as avoiding the 10+ instructions per allocation.

It’s entirely possible I’m wrong when it comes to Truffle, but at a minimum it seems like you would need arenas for each size class, and then you’d have to bump each arena by the number of local variables of that size class. The stack can do better than that.

0 comments

5 comments · 2 top-level

chrisseaton4y ago· 2 in thread

When you allocate objects individually it looks like this:

    object_a = tlab
    tlab += 8
    check tlab limit
    object_b = tlab
    tlab += 8
    check tlab limit

When you allocate as a single allocation it looks like this:

    object_a = tlab
    object_b = tlab + 8
    tlab += 16
    check tlab limit

What about that do you see as impossible?

> How could the GC possibly know where each object starts and ends if it did it as a single allocation?

As above.

coder543OP4y ago

In your example, the objects are all the same size. That would certainly be easy.

If you have three local objects that are 8 bytes, 16 bytes, and 32 bytes… if you do a single 48 byte allocation on the TLAB, how can the GC possibly know that there are three distinct objects, when it comes time to collect the garbage? I can think of a few ways to kind of make it work in a single buffer, but they would all require more than the 48 bytes that the objects themselves need. Separate TLAB arenas per size class seem like the best approach, but it would still require three allocations because each object is a different size.

I understand you’re some researcher related to Truffle… this is just the first I’m hearing of multiple object allocation being done in a single block with GC expected to do something useful.

chrisseaton4y ago

> If you have three local objects that are 8 bytes, 16 bytes, and 32 bytes… if you do a single 48 byte allocation on the TLAB, how can the GC possibly know that there are three distinct objects, when it comes time to collect the garbage?

Because the objects are self-describing - they have a class which tells you their size.

    object_a = tlab
    object_a.class = ClassA
    object_b = tlab + 8
    object_b.class = ClassB
    object_c = tlab + 16
    object_c.class = ClassC
    tlab += 48
    check tlab limit

1 more reply

chrisseaton4y ago· 1 in thread

> at a minimum it seems like you would need arenas for each size class

But TLABs are heterogenous by size. Objects of all sizes go in one linear allocation space. So allocating two objects next to each other is the same as allocating them both at the same time.

coder543OP4y ago

That doesn’t answer how the GC knows where the boundaries of each object are.

j / k navigate · click thread line to collapse

0 comments

5 comments · 2 top-level

chrisseaton4y ago· 2 in thread

When you allocate objects individually it looks like this:

    object_a = tlab
    tlab += 8
    check tlab limit
    object_b = tlab
    tlab += 8
    check tlab limit

When you allocate as a single allocation it looks like this:

    object_a = tlab
    object_b = tlab + 8
    tlab += 16
    check tlab limit

What about that do you see as impossible?

> How could the GC possibly know where each object starts and ends if it did it as a single allocation?

As above.

coder543OP4y ago

In your example, the objects are all the same size. That would certainly be easy.

I understand you’re some researcher related to Truffle… this is just the first I’m hearing of multiple object allocation being done in a single block with GC expected to do something useful.

chrisseaton4y ago

Because the objects are self-describing - they have a class which tells you their size.

    object_a = tlab
    object_a.class = ClassA
    object_b = tlab + 8
    object_b.class = ClassB
    object_c = tlab + 16
    object_c.class = ClassC
    tlab += 48
    check tlab limit

1 more reply

chrisseaton4y ago· 1 in thread

> at a minimum it seems like you would need arenas for each size class

But TLABs are heterogenous by size. Objects of all sizes go in one linear allocation space. So allocating two objects next to each other is the same as allocating them both at the same time.

coder543OP4y ago

That doesn’t answer how the GC knows where the boundaries of each object are.

j / k navigate · click thread line to collapse