franz1981 commented on a change in pull request #525: DISPATCH-1372 alloc_pool
intrusive linked list can be replaced by a linked stack
URL: https://github.com/apache/qpid-dispatch/pull/525#discussion_r297255900
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File path: src/alloc_pool.c
##########
@@ -164,14 +242,17 @@ void *qd_alloc(qd_alloc_type_desc_t *desc,
qd_alloc_pool_t **tpool)
desc->stats->held_by_threads += desc->config->transfer_batch_size;
#endif
for (idx = 0; idx < desc->config->transfer_batch_size; idx++) {
- item = DEQ_HEAD(desc->global_pool->free_list);
- DEQ_REMOVE_HEAD(desc->global_pool->free_list);
- DEQ_INSERT_TAIL(pool->free_list, item);
+ item = pop_stack(&desc->global_pool->free_list);
+ push_stack(&pool->free_list, item);
}
} else {
//
// Allocate a full batch from the heap and put it on the thread list.
//
+ //TODO(franz):
+ // - would be better to allocate in batches == transfer_batch_size
+ // and put a small (== sizeof(transfer_batch_size)) ref_count to
help the final free
+ // - could be beneficial directly to delink a chunk?
Review comment:
I need to explain it better IMHO. Let me rephrase something that I haven't
clarified yet on the JIRA.
The behaviour of this chunked stack is not actually the same of the original
one, in particular while "rebalancing" resources into/from the global free list.
I have chosen to just move `qd_alloc_item` between free lists, but not
`qd_alloc_chunk_t` on purpose, because:
- the purpose of `qd_alloc_chunk_t` is to cope with the allocation rate of
`qd_alloc_item` on a same thread
- to reduce the need to move "big" chunks between threads (causing
additional cache misses on the first attempt to access them)
- thread local lists are nearly bounded in size `localFreeListMax` ie can't
grow unbounded (maybe)
I'm not sure I've chosen right, that's the reason of the second comment ie
> could be beneficial directly to delink a chunk?
Maybe! I need a paper and a pencil to think about the implications of it and
which benefits will bring!
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