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https://issues.apache.org/jira/browse/DISPATCH-1372?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16872406#comment-16872406
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ASF GitHub Bot commented on DISPATCH-1372:
------------------------------------------
franz1981 commented on 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_r297228983
##########
File path: src/alloc_pool.c
##########
@@ -44,20 +46,97 @@ DEQ_DECLARE(qd_alloc_type_t, qd_alloc_type_list_t);
#define PATTERN_BACK 0xbabecafe
struct qd_alloc_item_t {
- DEQ_LINKS(qd_alloc_item_t);
uint32_t sequence;
#ifdef QD_MEMORY_DEBUG
qd_alloc_type_desc_t *desc;
uint32_t header;
#endif
};
-DEQ_DECLARE(qd_alloc_item_t, qd_alloc_item_list_t);
+#define CHUNK_SIZE 128/sizeof(void*)
+struct qd_alloc_chunk_t {
+ qd_alloc_chunk_t *prev;
+ qd_alloc_item_t *items[CHUNK_SIZE];
+ qd_alloc_chunk_t *next;
+};
+
+struct qd_alloc_linked_stack_t {
+ //the base
+ qd_alloc_chunk_t *top_chunk;
+ uint32_t top; //qd_alloc_item* top_item =
top_chunk->items[top+1] <-> top > 0
Review comment:
> to avoid the padding in the middle of the struct you want to put top at
the end of the struct.
I haven't considered field padding, you're right!
In this case, considering that `base_chunk` is > 128 bytes I won't risk to
get a cache miss on each access?
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> alloc_pool intrusive linked list can be replaced by a linked stack
> ------------------------------------------------------------------
>
> Key: DISPATCH-1372
> URL: https://issues.apache.org/jira/browse/DISPATCH-1372
> Project: Qpid Dispatch
> Issue Type: Improvement
> Components: Routing Engine
> Affects Versions: 1.8.0
> Reporter: Francesco Nigro
> Priority: Major
> Attachments: DOOM-3-BFG-Technical-Note.pdf,
> image-2019-06-21-11-08-17-015.png, image-2019-06-21-11-09-02-228.png,
> linked_list_misses.svg, stack_list_misses.svg
>
>
> alloc_pool is currently using a intrusive linked list approach to reduce the
> need of external data structures to hold data, saving expensive pointer
> chasing, but on modern architectures the data dependency between a current
> node and next/prev prevent the CPU prefetcher to stream nodes speculatively.
> There are different approaches that could benefit of prefetcing, but need to
> decouple the data stored from its container eg a linked stack.
> A linked stack is composed by doubly-linked chunks (allocated lazily) that
> make possible for the CPU to prefetch next/prev pointers given that those are
> already contained in the current chunk (if any).
> Although it seems counter-intuitive (given that introduce 1 more hop to reach
> the data), such data-structure is much more cache-friendly on modern
> architectures: I will attach some cache misses analysis to show it.
>
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