This sounds pretty cool.
On 13.05.2014 06:42, Adam Sakareassen via Digitalmars-d wrote:
Hi all,
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All memory allocations are entirely lock free (using CAS instructions). So a pre-empted thread will never block another. For allocations of less than 128 bytes, each thread is allocated memory from it's own memory pool to avoid false sharing on the CPU's cache.
There was a GSoC project a few years ago which was planning to do something similar, but then switched to go for implementing precise scanning. I always wanted to add lock-free memory allocations, too.
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The mark phase needs to stop the world. The sweeping portion of the collection will run in the background. This is similar to the current implementation as the world is restarted after the mark phase, however the thread doing the collection will not allocate the requested memory to the calling thread until after the sweep has completed. This means that single threaded applications always wait for the full garbage collection cycle.
Does this mean that destructors/finalizers are called from a different thread? It was never guaranteed to be run on the same thread as the allocation of the object so far, but effectively that happens for single-threaded applications. It could help to continue doing that, some resources have to be released from the same thread they have been acquired (e.g. UI handles). For multi-threaded applications, I'm fine with "guaranteeing" that the destructor is never run on the same thread, though some standard mechanism should exist to forward to another thread.
So far allocation speed seems to have improved. I can't test collection speed as it's not complete. As a test I wrote a simple function that allocates a linked list of 2 million items. This function is then spawned by 20 threads. This test script is shown below. Timing for allocation (with GC disabled) is as follows. (Using DMD 2.065) Existing GC code: 15700ms (average) My GC code: 500ms (Average)
Very promising numbers :-)
When performing the same amount of allocations on a single thread, the new code is still slightly faster than the old. What this demonstrates is that the locking mechanisms in the current GC code is a huge overhead for multi threaded applications that perform a lot of memory allocations. (ie. Use the “new” operator or dynamic arrays.) It would be nice to see the default GC and memory allocator improved. There is certainly room for improvement on the allocator end which may mask some of the performance issues associated with garbage collection. In the future I think D needs to look at making collection precise. It would not be too hard to adjust the mark and sweep GC to be nearly precise. The language needs to support precise GC before things like moving garbage collection become feasible.
You might check how compatible your implementation is with this: https://github.com/rainers/druntime/tree/gcx_precise2
