A new garbage collector

This is a new release of Church-State


New hashtable

In this release I have added a new hashtable implemented using the State language. Previously I was using a hashtable implemented in C from the kazlib library.

The new hashtable uses open addressing with double hashing to resolve probe collisions. Besides the standard hashtable which maps one key to one value, I have also implemented a two-key two-value hashtable used for the memoization of the OMeta parser results. For each grammar rule (key1) and the input position (key2) we store the result (value1) and the end position of the cached result (value 2). Previously I used two levels of hashtables for this.

You can see the hashtable here:


New garbage collector

I have also added a new garbage collector to replace the Boehm collector used so far. Like the Boehm collector this is a conservative mark and sweep collector.

The implementation is fairly simple, we create two heaps. The first is divided in pagelets which are 1024 bytes in size. Each of these pagelets can store objects which are 8,16,32,64,128,512 or 1024 bytes in size. We maintain bitfields to remember which objects are allocated within each page. When all the objects in a pagelet are freed during garbage collection that pagelet is returned to a free pool and may be used to store objects of a different size. As part of this work I have implemented a guard page and an associated signal handler to help detect when new free pagelets should be assigned to a size class.

The other heap is for objects larger than 1024 bytes. We use a simple first-fit strategy. To help mitigate the effect of the fragmentation of free spaces, we coalesce adjacent free spaces into larger blocks during the sweep phase.

My measurements show that the new allocator is about twice as slow as the Boehm allocator when allocating cons cells. For the entire bootstrap process the new allocator adds about 2 seconds of overhead, taking the bootstrap time from 23 to 25 seconds.

The new garbage collector is in state/runtime/state_runtime.state .