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Subject: memif insufficient padding
Date: Wed, 28 Aug 2024 23:04:51 +0200
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Thread-Topic: memif insufficient padding
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From: =?iso-8859-1?Q?Morten_Br=F8rup?= <mb@smartsharesystems.com>
To: "Jakub Grajciar" <jgrajcia@cisco.com>
Cc: <dev@dpdk.org>
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Jakub,

While browsing virtual interfaces in DPDK, I noticed a possible =
performance issue in the memif driver:

If "head" and "tail" are accessed by different lcores, they are not =
sufficiently far away from each other (and other hot fields) to prevent =
false sharing-like effects on systems with a next-N-lines hardware =
prefetcher, which will prefetch "tail" when fetching "head", and =
prefetch "head" when fetching "flags".

I suggest updating the structure somewhat like this:

-#define MEMIF_CACHELINE_ALIGN_MARK(mark) \
-	alignas(RTE_CACHE_LINE_SIZE) RTE_MARKER mark;
-
-typedef struct {
-	MEMIF_CACHELINE_ALIGN_MARK(cacheline0);
+typedef struct __rte_cache_aligned {
	uint32_t cookie;			/**< MEMIF_COOKIE */
	uint16_t flags;				/**< flags */
#define MEMIF_RING_FLAG_MASK_INT 1		/**< disable interrupt mode */
+	RTE_CACHE_GUARD; /* isolate head from flags */
	RTE_ATOMIC(uint16_t) head;			/**< pointer to ring buffer head */
-	MEMIF_CACHELINE_ALIGN_MARK(cacheline1);
+	RTE_CACHE_GUARD; /* isolate tail from head */
	RTE_ATOMIC(uint16_t) tail;			/**< pointer to ring buffer tail */
-	MEMIF_CACHELINE_ALIGN_MARK(cacheline2);
+	RTE_CACHE_GUARD; /* isolate descriptors from tail */
-	memif_desc_t desc[0];			/**< buffer descriptors */
+	memif_desc_t desc[];			/**< buffer descriptors */
} memif_ring_t;


Med venlig hilsen / Kind regards,
-Morten Br=F8rup