diff mbox

[v12,04/17] hbitmap: add hbitmap_merge

Message ID 1423532117-14490-5-git-send-email-jsnow@redhat.com
State New
Headers show

Commit Message

John Snow Feb. 10, 2015, 1:35 a.m. UTC 
We add a bitmap merge operation to assist in error cases
where we wish to combine two bitmaps together.

This is algorithmically O(bits) provided HBITMAP_LEVELS remains
constant. For a full bitmap on a 64bit machine:
sum(bits/64^k, k, 0, HBITMAP_LEVELS) ~= 1.01587 * bits

We may be able to improve running speed for particularly sparse
bitmaps by using iterators, but the running time for dense maps
will be worse.

We present the simpler solution first, and we can refine it later
if needed.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 include/qemu/hbitmap.h | 11 +++++++++++
 util/hbitmap.c         | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+)

Comments

Max Reitz Feb. 10, 2015, 10:16 p.m. UTC | #1 
On 2015-02-09 at 20:35, John Snow wrote:
> We add a bitmap merge operation to assist in error cases
> where we wish to combine two bitmaps together.
>
> This is algorithmically O(bits) provided HBITMAP_LEVELS remains
> constant. For a full bitmap on a 64bit machine:
> sum(bits/64^k, k, 0, HBITMAP_LEVELS) ~= 1.01587 * bits
>
> We may be able to improve running speed for particularly sparse
> bitmaps by using iterators, but the running time for dense maps
> will be worse.
>
> We present the simpler solution first, and we can refine it later
> if needed.
>
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>   include/qemu/hbitmap.h | 11 +++++++++++
>   util/hbitmap.c         | 32 ++++++++++++++++++++++++++++++++
>   2 files changed, 43 insertions(+)

Reviewed-by: Max Reitz <mreitz@redhat.com>
diff mbox

Patch

diff --git a/include/qemu/hbitmap.h b/include/qemu/hbitmap.h
index 550d7ce..c19c1cb 100644
--- a/include/qemu/hbitmap.h
+++ b/include/qemu/hbitmap.h
@@ -65,6 +65,17 @@  struct HBitmapIter {
 HBitmap *hbitmap_alloc(uint64_t size, int granularity);
 
 /**
+ * hbitmap_merge:
+ * @a: The bitmap to store the result in.
+ * @b: The bitmap to merge into @a.
+ *
+ * Merge two bitmaps together.
+ * A := A (BITOR) B.
+ * B is left unmodified.
+ */
+bool hbitmap_merge(HBitmap *a, const HBitmap *b);
+
+/**
  * hbitmap_empty:
  * @hb: HBitmap to operate on.
  *
diff --git a/util/hbitmap.c b/util/hbitmap.c
index ab13971..962ff29 100644
--- a/util/hbitmap.c
+++ b/util/hbitmap.c
@@ -395,3 +395,35 @@  HBitmap *hbitmap_alloc(uint64_t size, int granularity)
     hb->levels[0][0] |= 1UL << (BITS_PER_LONG - 1);
     return hb;
 }
+
+/**
+ * Given HBitmaps A and B, let A := A (BITOR) B.
+ * Bitmap B will not be modified.
+ */
+bool hbitmap_merge(HBitmap *a, const HBitmap *b)
+{
+    int i, j;
+    uint64_t size;
+
+    if ((a->size != b->size) || (a->granularity != b->granularity)) {
+        return false;
+    }
+
+    if (hbitmap_count(b) == 0) {
+        return true;
+    }
+
+    /* This merge is O(size), as BITS_PER_LONG and HBITMAP_LEVELS are constant.
+     * It may be possible to improve running times for sparsely populated maps
+     * by using hbitmap_iter_next, but this is suboptimal for dense maps.
+     */
+    size = a->size;
+    for (i = HBITMAP_LEVELS - 1; i >= 0; i--) {
+        size = MAX((size + BITS_PER_LONG - 1) >> BITS_PER_LEVEL, 1);
+        for (j = 0; j < size; j++) {
+            a->levels[i][j] |= b->levels[i][j];
+        }
+    }
+
+    return true;
+}