svn commit: r320527 - head/sys/kern
Alan Cox
alc at FreeBSD.org
Sat Jul 1 05:27:41 UTC 2017
Author: alc
Date: Sat Jul 1 05:27:40 2017
New Revision: 320527
URL: https://svnweb.freebsd.org/changeset/base/320527
Log:
Change blst_leaf_alloc() to handle a cursor argument, and to improve
performance.
To find in the leaf bitmap all ranges of sufficient length, use a doubling
strategy with shift-and-and until each bit still set represents a bit
sequence of length 'count', or until the bitmask is zero. In the latter
case, update the hint based on the first bit sequence length not found to
be available. For example, seeking an interval of length 12, the set bits
of the bitmap would represent intervals of length 1, then 2, then 3, then
6, then 12. If no bits are set at the point when each bit represents an
interval of length 6, then the hint can be updated to 5 and the search
terminated.
If long-enough intervals are found, discard those before the cursor. If
any remain, use binary search to find the position of the first of them,
and allocate that interval.
Submitted by: Doug Moore <dougm at rice.edu>
Reviewed by: kib, markj
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D11426
Modified:
head/sys/kern/subr_blist.c
Modified: head/sys/kern/subr_blist.c
==============================================================================
--- head/sys/kern/subr_blist.c Sat Jul 1 03:39:38 2017 (r320526)
+++ head/sys/kern/subr_blist.c Sat Jul 1 05:27:40 2017 (r320527)
@@ -121,7 +121,8 @@ void panic(const char *ctl, ...);
* static support functions
*/
-static daddr_t blst_leaf_alloc(blmeta_t *scan, daddr_t blk, int count);
+static daddr_t blst_leaf_alloc(blmeta_t *scan, daddr_t blk, int count,
+ daddr_t cursor);
static daddr_t blst_meta_alloc(blmeta_t *scan, daddr_t blk, daddr_t count,
daddr_t radix, daddr_t skip, daddr_t cursor);
static void blst_leaf_free(blmeta_t *scan, daddr_t relblk, int count);
@@ -227,7 +228,8 @@ blist_alloc(blist_t bl, daddr_t count)
*/
while (count <= bl->bl_root->bm_bighint) {
if (bl->bl_radix == BLIST_BMAP_RADIX)
- blk = blst_leaf_alloc(bl->bl_root, 0, count);
+ blk = blst_leaf_alloc(bl->bl_root, 0, count,
+ bl->bl_cursor);
else
blk = blst_meta_alloc(bl->bl_root, 0, count,
bl->bl_radix, bl->bl_skip, bl->bl_cursor);
@@ -352,77 +354,92 @@ blist_print(blist_t bl)
/*
* blist_leaf_alloc() - allocate at a leaf in the radix tree (a bitmap).
*
- * This is the core of the allocator and is optimized for the 1 block
- * and the BLIST_BMAP_RADIX block allocation cases. Other cases are
- * somewhat slower. The 1 block allocation case is log2 and extremely
- * quick.
+ * This is the core of the allocator and is optimized for the
+ * BLIST_BMAP_RADIX block allocation case. Otherwise, execution
+ * time is proportional to log2(count) + log2(BLIST_BMAP_RADIX).
*/
static daddr_t
-blst_leaf_alloc(
- blmeta_t *scan,
- daddr_t blk,
- int count
-) {
- u_daddr_t orig = scan->u.bmu_bitmap;
+blst_leaf_alloc(blmeta_t *scan, daddr_t blk, int count, daddr_t cursor)
+{
+ u_daddr_t mask;
+ int count1, hi, lo, mid, num_shifts, range1, range_ext;
- if (orig == 0) {
+ if (count == BLIST_BMAP_RADIX) {
/*
- * Optimize bitmap all-allocated case. Also, count = 1
- * case assumes at least 1 bit is free in the bitmap, so
- * we have to take care of this case here.
+ * Optimize allocation of BLIST_BMAP_RADIX bits. If this wasn't
+ * a special case, then forming the final value of 'mask' below
+ * would require special handling to avoid an invalid left shift
+ * when count equals the number of bits in mask.
*/
+ if (~scan->u.bmu_bitmap != 0) {
+ scan->bm_bighint = BLIST_BMAP_RADIX - 1;
+ return (SWAPBLK_NONE);
+ }
+ if (cursor != blk)
+ return (SWAPBLK_NONE);
+ scan->u.bmu_bitmap = 0;
scan->bm_bighint = 0;
- return(SWAPBLK_NONE);
+ return (blk);
}
- if (count == 1) {
+ range1 = 0;
+ count1 = count - 1;
+ num_shifts = fls(count1);
+ mask = scan->u.bmu_bitmap;
+ while (mask != 0 && num_shifts > 0) {
/*
- * Optimized code to allocate one bit out of the bitmap
+ * If bit i is set in mask, then bits in [i, i+range1] are set
+ * in scan->u.bmu_bitmap. The value of range1 is equal to
+ * count1 >> num_shifts. Grow range and reduce num_shifts to 0,
+ * while preserving these invariants. The updates to mask leave
+ * fewer bits set, but each bit that remains set represents a
+ * longer string of consecutive bits set in scan->u.bmu_bitmap.
*/
- u_daddr_t mask;
- int j = BLIST_BMAP_RADIX/2;
- int r = 0;
-
- mask = (u_daddr_t)-1 >> (BLIST_BMAP_RADIX/2);
-
- while (j) {
- if ((orig & mask) == 0) {
- r += j;
- orig >>= j;
- }
- j >>= 1;
- mask >>= j;
- }
- scan->u.bmu_bitmap &= ~((u_daddr_t)1 << r);
- return(blk + r);
+ num_shifts--;
+ range_ext = range1 + ((count1 >> num_shifts) & 1);
+ mask &= mask >> range_ext;
+ range1 += range_ext;
}
- if (count <= BLIST_BMAP_RADIX) {
+ if (mask == 0) {
/*
- * non-optimized code to allocate N bits out of the bitmap.
- * The more bits, the faster the code runs. It will run
- * the slowest allocating 2 bits, but since there aren't any
- * memory ops in the core loop (or shouldn't be, anyway),
- * you probably won't notice the difference.
+ * Update bighint. There is no allocation bigger than range1
+ * available in this leaf.
*/
- int j;
- int n = BLIST_BMAP_RADIX - count;
- u_daddr_t mask;
+ scan->bm_bighint = range1;
+ return (SWAPBLK_NONE);
+ }
- mask = (u_daddr_t)-1 >> n;
+ /*
+ * Discard any candidates that appear before the cursor.
+ */
+ lo = cursor - blk;
+ mask &= ~(u_daddr_t)0 << lo;
- for (j = 0; j <= n; ++j) {
- if ((orig & mask) == mask) {
- scan->u.bmu_bitmap &= ~mask;
- return(blk + j);
- }
- mask = (mask << 1);
- }
+ if (mask == 0)
+ return (SWAPBLK_NONE);
+
+ /*
+ * The least significant set bit in mask marks the start of the first
+ * available range of sufficient size. Clear all the bits but that one,
+ * and then perform a binary search to find its position.
+ */
+ mask &= -mask;
+ hi = BLIST_BMAP_RADIX - count1;
+ while (lo + 1 < hi) {
+ mid = (lo + hi) >> 1;
+ if ((mask >> mid) != 0)
+ lo = mid;
+ else
+ hi = mid;
}
+
/*
- * We couldn't allocate count in this subtree, update bighint.
+ * Set in mask exactly the bits being allocated, and clear them from
+ * the set of available bits.
*/
- scan->bm_bighint = count - 1;
- return(SWAPBLK_NONE);
+ mask = (mask << count) - mask;
+ scan->u.bmu_bitmap &= ~mask;
+ return (blk + lo);
}
/*
@@ -491,7 +508,8 @@ blst_meta_alloc(blmeta_t *scan, daddr_t blk, daddr_t c
* The allocation might fit in the i'th subtree.
*/
if (next_skip == 1) {
- r = blst_leaf_alloc(&scan[i], blk, count);
+ r = blst_leaf_alloc(&scan[i], blk, count,
+ cursor > blk ? cursor : blk);
} else {
r = blst_meta_alloc(&scan[i], blk, count,
radix, next_skip - 1, cursor > blk ?
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