git: 3684bb89d520 - stable/13 - MFC: Improve readahead in loader

The branch stable/13 has been updated by cperciva:

URL: https://cgit.FreeBSD.org/src/commit/?id=3684bb89d5208575b341833225a798d5b5c465de

commit 3684bb89d5208575b341833225a798d5b5c465de
Author:     Colin Percival <cperciva@FreeBSD.org>
AuthorDate: 2021-09-19 22:24:00 +0000
Commit:     Colin Percival <cperciva@FreeBSD.org>
CommitDate: 2021-12-29 22:53:19 +0000

    MFC: Improve readahead in loader
    
    style: Fix leading whitespace in bcache.c
    loader: Refactor readahead adjustment in bcache
    loader bcache: Track unconsumed readahead
    loader bcache: Allow readahead up to 256 kB I/Os
    
    Sponsored by:   https://www.patreon.com/cperciva
    
    (cherry picked from commit 5a01dea7e8c9640605e5731a9fc7f600c07ace61)
    (cherry picked from commit b841148bbbdc967c871e8742a6f0b7b17b2d1d91)
    (cherry picked from commit 04b9b7c507c52daf7b2999329a50825db098151f)
    (cherry picked from commit 248682a589159619aa5f2019e415a423e849e327)
---
 stand/common/bcache.c | 40 +++++++++++++++++++++++++++++++++-------
 1 file changed, 33 insertions(+), 7 deletions(-)

diff --git a/stand/common/bcache.c b/stand/common/bcache.c
index 526f9fe3fa5c..99c78b0346b7 100644
--- a/stand/common/bcache.c
+++ b/stand/common/bcache.c
@@ -67,6 +67,7 @@ struct bcache {
     size_t		bcache_nblks;
     size_t		ra;
     daddr_t		bcache_nextblkno;
+    size_t		ralen;
 };
 
 static u_int bcache_total_nblks;	/* set by bcache_init */
@@ -85,8 +86,9 @@ static u_int bcache_rablks;
 #define	BHASH(bc, blkno)	((blkno) & ((bc)->bcache_nblks - 1))
 #define	BCACHE_LOOKUP(bc, blkno)	\
 	((bc)->bcache_ctl[BHASH((bc), (blkno))].bc_blkno != (blkno))
-#define	BCACHE_READAHEAD	256
+#define	BCACHE_READAHEAD	512
 #define	BCACHE_MINREADAHEAD	32
+#define	BCACHE_MAXIOWRA		512
 
 static void	bcache_invalidate(struct bcache *bc, daddr_t blkno);
 static void	bcache_insert(struct bcache *bc, daddr_t blkno);
@@ -238,20 +240,39 @@ read_strategy(void *devdata, int rw, daddr_t blk, size_t size,
 	if (BCACHE_LOOKUP(bc, (daddr_t)(blk + i))) {
 	    bcache_misses += (nblk - i);
 	    complete = 0;
-	    if (nblk - i > BCACHE_MINREADAHEAD && bc->ra > BCACHE_MINREADAHEAD)
-		bc->ra >>= 1;	/* reduce read ahead */
 	    break;
 	} else {
 	    bcache_hits++;
 	}
     }
 
-   if (complete) {	/* whole set was in cache, return it */
+    /*
+     * Adjust read-ahead size if appropriate.  Subject to the requirement
+     * that bc->ra must stay in between MINREADAHEAD and READAHEAD, we
+     * increase it when we notice that readahead was useful and decrease
+     * it when we notice that readahead was not useful.
+     */
+    if (complete || (i == bc->ralen && bc->ralen > 0)) {
 	if (bc->ra < BCACHE_READAHEAD)
-		bc->ra <<= 1;	/* increase read ahead */
+	    bc->ra <<= 1;	/* increase read ahead */
+    } else {
+	if (nblk - i > BCACHE_MINREADAHEAD && bc->ralen > 0 &&
+	  bc->ra > BCACHE_MINREADAHEAD)
+	    bc->ra >>= 1;	/* reduce read ahead */
+    }
+
+    /* Adjust our "unconsumed readahead" value. */
+    if (blk == bc->bcache_nextblkno) {
+	if (nblk > bc->ralen)
+	    bc->ralen = 0;
+	else
+	    bc->ralen -= nblk;
+    }
+
+    if (complete) {	/* whole set was in cache, return it */
 	bcopy(bc->bcache_data + (bcache_blksize * BHASH(bc, blk)), buf, size);
 	goto done;
-   }
+    }
 
     /*
      * Fill in any misses. From check we have i pointing to first missing
@@ -304,7 +325,12 @@ read_strategy(void *devdata, int rw, daddr_t blk, size_t size,
     if (ra != 0 && ra != bc->bcache_nblks) { /* do we have RA space? */
 	ra = MIN(bc->ra, ra - 1);
 	ra = rounddown(ra, 16);		/* multiple of 16 blocks */
+	if (ra + p_size > BCACHE_MAXIOWRA)
+	    ra = BCACHE_MAXIOWRA - p_size;
+	bc->ralen = ra;
 	p_size += ra;
+    } else {
+	bc->ralen = 0;
     }
 
     /* invalidate bcache */
@@ -351,7 +377,7 @@ read_strategy(void *devdata, int rw, daddr_t blk, size_t size,
 	result = 0;
     }
 
- done:
+done:
     if (result == 0) {
         if (rsize != NULL)
 	    *rsize = size;