Decouple arc_read_done callback from arc buf instantiation

Add ARC_FLAG_NO_BUF to indicate that a buffer need not be instantiated. This fixes a ~20% performance regression on cached reads due to zfetch changes. Reviewed-by: Tony Nguyen <[email protected]> Reviewed-by: Brian Behlendorf <[email protected]> Reviewed-by: Matthew Ahrens <[email protected]> Signed-off-by: Matt Macy <[email protected]> Closes #11220 Closes #11232
openzfs · Dec 9, 2020 · 1e4732c · 1e4732c
1 parent edb20ff
commit 1e4732c
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Showing 3 changed files with 10 additions and 3 deletions.
diff --git a/include/sys/arc.h b/include/sys/arc.h
@@ -154,6 +154,11 @@ typedef enum arc_flags
 	 */
 	ARC_FLAG_CACHED_ONLY		= 1 << 22,
 
+	/*
+	 * Don't instantiate an arc_buf_t for arc_read_done.
+	 */
+	ARC_FLAG_NO_BUF			= 1 << 23,
+
 	/*
 	 * The arc buffer's compression mode is stored in the top 7 bits of the
 	 * flags field, so these dummy flags are included so that MDB can

diff --git a/module/zfs/arc.c b/module/zfs/arc.c
@@ -5924,6 +5924,7 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 	boolean_t noauth_read = BP_IS_AUTHENTICATED(bp) &&
 	    (zio_flags & ZIO_FLAG_RAW_ENCRYPT) != 0;
 	boolean_t embedded_bp = !!BP_IS_EMBEDDED(bp);
+	boolean_t no_buf = *arc_flags & ARC_FLAG_NO_BUF;
 	int rc = 0;
 
 	ASSERT(!embedded_bp ||
@@ -5998,7 +5999,7 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 			}
 			ASSERT(*arc_flags & ARC_FLAG_NOWAIT);
 
-			if (done) {
+			if (done && !no_buf) {
 				arc_callback_t *acb = NULL;
 
 				acb = kmem_zalloc(sizeof (arc_callback_t),
@@ -6027,7 +6028,7 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 		ASSERT(hdr->b_l1hdr.b_state == arc_mru ||
 		    hdr->b_l1hdr.b_state == arc_mfu);
 
-		if (done) {
+		if (done && !no_buf) {
 			if (hdr->b_flags & ARC_FLAG_PREDICTIVE_PREFETCH) {
 				/*
 				 * This is a demand read which does not have to

diff --git a/module/zfs/dbuf.c b/module/zfs/dbuf.c
@@ -3040,7 +3040,8 @@ dbuf_issue_final_prefetch(dbuf_prefetch_arg_t *dpa, blkptr_t *bp)
 
 	int zio_flags = ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE;
 	arc_flags_t aflags =
-	    dpa->dpa_aflags | ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH;
+	    dpa->dpa_aflags | ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH |
+	    ARC_FLAG_NO_BUF;
 
 	/* dnodes are always read as raw and then converted later */
 	if (BP_GET_TYPE(bp) == DMU_OT_DNODE && BP_IS_PROTECTED(bp) &&