Am 08.06.2013 09:50, schrieb Rainer Schuetze:
On 06.06.2013 22:27, Benjamin Thaut wrote:
Am 06.06.2013 08:28, schrieb Rainer Schuetze:
On 05.06.2013 16:14, bearophile wrote:
Andrei Alexandrescu:
http://www.reddit.com/r/programming/comments/1fpw2r/dconf_2013_day_2_talk_5_a_precise_garbage/
Is this useful to make the GC precise regarding the stack too?
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.19.5570
I was imagining something similar too. The part about multi-threading in
the paper isn't too convincing, though. Especially the need for GC safe
points in tight loops to block a thread during collection will not get
many friends in the D community.
I think you don't really need percise scanning which is thread safe. If
you have one pool per thread, and you can scan the thread local pools
percisley within the thread that would be enough. Because you would then
be able to do generational garbage collection for the thread local
pools. If you have to scan one of (or the) global pool, percise scanning
of the stacks is not really needed, the old impercises scanning is
sufficient, you just have to pin those memory blocks you might think are
referenced from stack memory.
Wouldn't that mean a write-barrier for every pointer assignment?
Not for every pointer assignment, only for pointers that are on the
heap. Also most of these write barries could be skipped with really
cheap tests most of the time.
But to be able to actually do thread local pools a bit more then write
barriers would be needed. For each of the following situations a call
into druntime would be needed.
1) Creating a shared or immutable object
2) Casting to shared or immutable
3) Assigning a shared, immutable, __gshared global or static variable
Considering "string" is "immutable(char)[]", would you want to allocate
all temporary strings on the global heap? Also, I don't like to have
possible expensive operations for casting.
Good point, didn't think about that yet. In Order to improve the
performance of the GC we have to sacrifice performance in other places
of the language in my opinion. Given the fact that the current GC is 3
times slower then manual memory mangement (in my test case). It should
be easly possible to take a performance hit here and there but end up
beeing faster in the end anyway. The current GC is especially bad with
short lived allocations and I don't think this is going to get any
better when only using a Mark & Sweep. The D community has to see that
to get a state of the art GC some perfomance sacrifces have to be done.
If those are not wanted we can go back to manual memory management right
away, in my opinion. I recently also had a long talk with Claus
Gittinger (created the SmalltalkX VM) about the topic. He also had the
same opinion on the topic. He also stated that it is going to be very
hard if not impossible to implement a state of the Art GC into a
language like D which does not have any restrictions in the language
which help with that Task.
If you have these and you are able to percisley scan the stack for the
current thread only you will then be able to move all of the referenced
memory from the thread local pool to the global pool if any of the above
calls happen.
This would mean that most of the time only thread local garbage
collection would be neccessary and you won't have to stop other threads
from doing whatever they are doing. Only in rare cases it should be
necessary to scan the global pool.
I agree that a thread local pool can give good performance improvements.
But as long as you still have a global heap (which you probably cannot
eliminate), it's not a simplification to have thread local garbage
collections in addition.
No one said that it is a simplification. Its always going to get more
complicated if you want a better GC.
The problem to implement it is that shared semantics are still pretty
undefined. AFAICT "shared" is only a type modifier that has different
conversion rules than non-shared types. There are no runtime guarantees
with "shared", even less with the absence of shared, and even if they
exist, __gshared and casting are meant to subvert them.
Then maybe the semantics of shared should be defined with a GC in mind.
