Hi all,
As a learning exercise I've just been doing some experimenting with
rewriting the garbage collection code, and thought I might share some of
the initial results. I only program as a hobby these days, and I'm
certainly no expert, but I thought some people might find it interesting.
My interest started because I wrote a LR1 file parser in D. I then
multi threaded the application so multiple files could be parsed
simultaneously. (Disk IO was all on one thread). To my surprise, the
through-put dropped significantly. I could process the files a lot
faster using only one thread. It turns out the delays were due to my
liberal use of the “new” statement. Rather than block allocate the
memory I thought I would just hack the GC. So I cleared out gc.d in the
runtime and started again. The basic plan was to make it more
multi-thread friendly. Already I have learnt quite a lot. There are a
number of things I would do differently if I tried another re-write.
However so far it has been a good learning experience.
Currently, allocations are all working, and the mark phase is running.
It still will not sweep and free the memory.
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. The collector
component runs on a background thread using a mark and sweep algorithm
which is basically the same as the existing algorithm. Currently the
thread will wake up every 100ms and decide if a collection should be
performed. An emergency collection will run in the foreground if a
memory allocation fails during that period.
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.
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)
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.
Anyway, I just thought I'd share the results of my experimenting. I
would be happy to make the code available in a few weeks time. Perhaps
someone might find is useful. I need to get it finished and tested
first. :-)
Cheers!
Adam
------
//Test script that generated these results:
import std.stdio;
import std.datetime;
import std.concurrency;
import core.memory;
class LinkedList{
long value =0;
LinkedList next;
}
shared int threadCount = 0;
void main(){
core.memory.GC.disable();
auto start = Clock.currSystemTick();
foreach(i; 0 .. 20){
auto tid = spawn(&doSomething, thisTid);
threadCount++;
}
while(threadCount >0){};
auto ln = Clock.currSystemTick() - start;
writeln(ln.msecs, "ms");
}
void doSomething(Tid tid){
auto top = new LinkedList;
auto recent = top;
//Create the linked list
foreach(i; 1 .. 2_000_000){
auto newList = new LinkedList;
newList.value = i;
recent.next = newList;
recent = newList;
}
//Sum the values. (Just spends some time walking the memory).
recent = top;
long total=0;
while(recent !is null){
total += recent.value;
recent = recent.next;
}
writeln("Total : ", total );
threadCount--;
}
