From a98fe72db88350fcec030487208e6c50c9de1974 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Mon, 12 Feb 2024 13:26:10 -0500
Subject: ioq: Get rid of IOQ_STRIDE
Benchmarks show it hurts more than it helps.
---
src/ioq.c | 25 +++++--------------------
1 file changed, 5 insertions(+), 20 deletions(-)
(limited to 'src')
diff --git a/src/ioq.c b/src/ioq.c
index 50550ed..438601d 100644
--- a/src/ioq.c
+++ b/src/ioq.c
@@ -26,15 +26,11 @@
*
* Pushes are implemented with an unconditional
*
- * fetch_add(&ioqq->head, IOQ_STRIDE)
+ * fetch_add(&ioqq->head, 1)
*
* which scales better on many architectures than compare-and-swap (see [1] for
- * details). Pops are implemented similarly. We add IOQ_STRIDE rather than 1
- * so that successive queue elements are on different cache lines, but the
- * exposition below uses 1 for simplicity.
- *
- * Since the fetch-and-adds are unconditional, non-blocking readers can get
- * ahead of writers:
+ * details). Pops are implemented similarly. Since the fetch-and-adds are
+ * unconditional, non-blocking readers can get ahead of writers:
*
* Reader Writer
* ──────────────── ──────────────────────
@@ -204,17 +200,6 @@ struct ioqq {
cache_align ioq_slot slots[];
};
-// If we assign slots sequentially, threads will likely be operating on
-// consecutive slots. If these slots are in the same cache line, that will
-// result in false sharing. We can mitigate this by assigning slots with a
-// stride larger than a cache line e.g. 0, 9, 18, ..., 1, 10, 19, ...
-// As long as the stride is relatively prime to circular buffer length, we'll
-// still use every available slot. Since the length is a power of two, that
-// means the stride must be odd.
-
-#define IOQ_STRIDE ((FALSE_SHARING_SIZE / sizeof(ioq_slot)) | 1)
-bfs_static_assert(IOQ_STRIDE % 2 == 1);
-
/** Destroy an I/O command queue. */
static void ioqq_destroy(struct ioqq *ioqq) {
if (!ioqq) {
@@ -357,7 +342,7 @@ static bool ioq_slot_push(struct ioqq *ioqq, ioq_slot *slot, struct ioq_ent *ent
/** Push an entry onto the queue. */
static void ioqq_push(struct ioqq *ioqq, struct ioq_ent *ent) {
while (true) {
- size_t i = fetch_add(&ioqq->head, IOQ_STRIDE, relaxed);
+ size_t i = fetch_add(&ioqq->head, 1, relaxed);
ioq_slot *slot = &ioqq->slots[i & ioqq->slot_mask];
if (ioq_slot_push(ioqq, slot, ent)) {
break;
@@ -400,7 +385,7 @@ static struct ioq_ent *ioq_slot_pop(struct ioqq *ioqq, ioq_slot *slot, bool bloc
/** Pop an entry from the queue. */
static struct ioq_ent *ioqq_pop(struct ioqq *ioqq, bool block) {
- size_t i = fetch_add(&ioqq->tail, IOQ_STRIDE, relaxed);
+ size_t i = fetch_add(&ioqq->tail, 1, relaxed);
ioq_slot *slot = &ioqq->slots[i & ioqq->slot_mask];
return ioq_slot_pop(ioqq, slot, block);
}
--
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