Multi-way merges work fine in simple cases.

I am going to give them a try on large data to see if they are viable.

diff --git a/lib/sorter/common.h b/lib/sorter/common.h
index db414dd08c6c033e99124ce00b373b68997a3881..8b87ed90b4414b9e8c46374c09fdf84e80fd10ec 100644 (file)
--- a/lib/sorter/common.h
+++ b/lib/sorter/common.h
@@ -26,6 +26,7 @@ enum sort_debug {
   SORT_DEBUG_NO_JOIN = 2,
   SORT_DEBUG_KEEP_BUCKETS = 4,
   SORT_DEBUG_NO_RADIX = 8,
+  SORT_DEBUG_NO_MULTIWAY = 16,
 };
 
 struct sort_bucket;
@@ -54,6 +55,9 @@ struct sort_context {
   // Bucket arrays are NULL-terminated.
   void (*twoway_merge)(struct sort_context *ctx, struct sort_bucket **ins, struct sort_bucket **outs);
 
+  // Multi-way merge: merge an arbitrary number of source buckets to a single destination bucket.
+  void (*multiway_merge)(struct sort_context *ctx, struct sort_bucket **ins, struct sort_bucket *out);
+
   // Radix split according to hash function
   void (*radix_split)(struct sort_context *ctx, struct sort_bucket *in, struct sort_bucket **outs, uns bitpos, uns numbits);
 

diff --git a/lib/sorter/govern.c b/lib/sorter/govern.c
index 6b4dc696ca06de80d4bce4e3c6067f99133fa647..745f6b05b1cc95cce0371350959c02b2a4c00baf 100644 (file)
--- a/lib/sorter/govern.c
+++ b/lib/sorter/govern.c
@@ -177,6 +177,90 @@ sorter_twoway(struct sort_context *ctx, struct sort_bucket *b)
   clist_insert_after(&ins[0]->n, list_pos);
 }
 
+static void
+sorter_multiway(struct sort_context *ctx, struct sort_bucket *b)
+{
+  clist parts;
+  cnode *list_pos = b->n.prev;
+  struct sort_bucket *join = sbuck_join_to(b);
+
+  clist_init(&parts);
+  ASSERT(!(sorter_debug & SORT_DEBUG_NO_PRESORT));
+  // FIXME: What if the parts will be too small?
+  SORT_XTRACE(3, "%s", ((b->flags & SBF_CUSTOM_PRESORT) ? "Custom presorting" : "Presorting"));
+  uns cont;
+  uns part_cnt = 0;
+  u64 total_size = 0;
+  sorter_start_timer(ctx);
+  do
+    {
+      struct sort_bucket *p = sbuck_new(ctx);
+      clist_add_tail(&parts, &p->n);
+      cont = sorter_presort(ctx, b, p, (!part_cnt && join) ? join : p);
+      part_cnt++;
+      total_size += sbuck_size(p);
+    }
+  while (cont);
+  sorter_stop_timer(ctx, &ctx->total_pre_time);
+
+  // FIXME: This is way too similar to the two-way case.
+  if (part_cnt == 1)
+    {
+      struct sort_bucket *p = clist_head(&parts);
+      SORT_XTRACE(((b->flags & SBF_SOURCE) ? 1 : 2), "Sorted in memory");
+      if (join)
+       {
+         ASSERT(join->runs == 2);
+         join->runs--;
+         sbuck_drop(b);
+       }
+      else
+       clist_insert_after(&p->n, list_pos);
+      sbuck_drop(b);
+      return;
+    }
+
+  SORT_TRACE("Multi-way presorting pass (%d parts, %s, %dMB/s)", part_cnt, stk_fsize(total_size), sorter_speed(ctx, total_size));
+  sbuck_drop(b);
+
+  uns max_ways = 16;
+  struct sort_bucket *ways[max_ways+1];
+  SORT_XTRACE(2, "Starting up to %d-way merge", max_ways);
+  for (;;)
+    {
+      uns n = 0;
+      struct sort_bucket *p;
+      while (n < max_ways && (p = clist_head(&parts)))
+       {
+         clist_remove(&p->n);
+         ways[n++] = p;
+       }
+      ways[n] = NULL;
+      ASSERT(n > 1);
+
+      struct sort_bucket *out;
+      out = sbuck_new(ctx);            // FIXME: No joining so far
+      sorter_start_timer(ctx);
+      ctx->multiway_merge(ctx, ways, out);
+      sorter_stop_timer(ctx, &ctx->total_ext_time);
+
+      for (uns i=0; i<n; i++)
+       sbuck_drop(ways[i]);
+
+      if (clist_empty(&parts))
+       {
+         clist_insert_after(&out->n, list_pos);
+         SORT_TRACE("Multi-way merge completed (%s, %dMB/s)", F_BSIZE(out), sorter_speed(ctx, sbuck_size(out)));
+         return;
+       }
+      else
+       {
+         clist_add_tail(&parts, &out->n);
+         SORT_TRACE("Multi-way merge pass (%d ways, %s, %dMB/s)", n, F_BSIZE(out), sorter_speed(ctx, sbuck_size(out)));
+       }
+    }
+}
+
 static uns
 sorter_radix_bits(struct sort_context *ctx, struct sort_bucket *b)
 {
@@ -271,6 +355,8 @@ sorter_run(struct sort_context *ctx)
        sbuck_drop(b);
       else if (b->runs == 1)
        sorter_join(b);
+      else if (ctx->multiway_merge && !(sorter_debug & (SORT_DEBUG_NO_MULTIWAY | SORT_DEBUG_NO_PRESORT)))      // FIXME: Just kidding...
+       sorter_multiway(ctx, b);
       else if (bits = sorter_radix_bits(ctx, b))
        sorter_radix(ctx, b, bits);
       else

diff --git a/lib/sorter/s-multiway.h b/lib/sorter/s-multiway.h
index a4352fe96c819401ff0bd7ce1ce150b7ee92d569..32956e700f61b30a440d3b34523ec3a25ca749a4 100644 (file)
--- a/lib/sorter/s-multiway.h
+++ b/lib/sorter/s-multiway.h
@@ -22,8 +22,12 @@ static inline void P(update_tree)(P(key) *keys, int *tree, uns i)
     }
 }
 
-static void P(multiway_merge)(struct sort_context *ctx UNUSED, struct sort_bucket **ins, uns num_ins, struct sort_bucket *out)
+static void P(multiway_merge)(struct sort_context *ctx UNUSED, struct sort_bucket **ins, struct sort_bucket *out)
 {
+  uns num_ins = 0;
+  while (ins[num_ins])
+    num_ins++;
+
   uns n2 = 1;
   while (n2 < num_ins)
     n2 *= 2;
@@ -32,7 +36,7 @@ static void P(multiway_merge)(struct sort_context *ctx UNUSED, struct sort_bucke
   struct fastbuf *fins[num_ins];
   P(key) keys[num_ins];                // FIXME: Tune num_ins according to L1 cache size
   int tree[2*n2];              // A complete binary tree, leaves are input streams, each internal vertex contains a minimum of its sons
-  for (uns i=1; i<=n2; i++)
+  for (uns i=1; i<2*n2; i++)
     tree[i] = -1;
 
   for (uns i=0; i<num_ins; i++)

diff --git a/lib/sorter/sorter.h b/lib/sorter/sorter.h
index e329fa29447385c3e14269f470b9a26a43584f13..a0b6b39b76a1407fa7425f300bd0551b75cbac76 100644 (file)
--- a/lib/sorter/sorter.h
+++ b/lib/sorter/sorter.h
@@ -201,6 +201,10 @@ static inline void P(copy_merged)(P(key) **keys, struct fastbuf **data UNUSED, u
 
 #include "lib/sorter/s-twoway.h"
 
+#ifndef SORT_UNIFY
+#include "lib/sorter/s-multiway.h"
+#endif
+
 #if defined(SORT_HASH_BITS) || defined(SORT_INT)
 #include "lib/sorter/s-radix.h"
 #endif
@@ -269,6 +273,10 @@ static struct fastbuf *P(sort)(
   ctx.internal_estimate = P(internal_estimate);
   ctx.twoway_merge = P(twoway_merge);
 
+#ifndef SORT_UNIFY
+  ctx.multiway_merge = P(multiway_merge);
+#endif
+
   sorter_run(&ctx);
 
 #ifdef SORT_OUTPUT_FILE