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From 5454ced7c8cfc2ba278c2635eecb9a5e4841e613 Mon Sep 17 00:00:00 2001
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From: Nir Soffer <nsoffer@redhat.com>
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Date: Fri, 5 Nov 2021 22:16:26 +0200
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Subject: [PATCH] common/urils/vector.c: Optimize vector append
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Minimize reallocs by growing the backing array by factor of 1.5.
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Testing show that now append() is fast without calling reserve()
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upfront, simplifying code using vector.
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NBDKIT_BENCH=1 ./test-vector
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bench_reserve: 1000000 appends in 0.004496 s
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bench_append: 1000000 appends in 0.004180 s
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This can make a difference in code appending millions of items.
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Ported from libnbd commit 985dfa72ae2e41901f0af21e7205ef85428cd4bd.
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(cherry picked from commit 12356fa97a840de19bb61e0abedd6e7c7e578e5a)
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---
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common/utils/vector.c | 14 ++++++++++++--
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1 file changed, 12 insertions(+), 2 deletions(-)
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diff --git a/common/utils/vector.c b/common/utils/vector.c
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index 00cd2546..7df17e1b 100644
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--- a/common/utils/vector.c
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+++ b/common/utils/vector.c
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@@ -41,11 +41,21 @@ int
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generic_vector_reserve (struct generic_vector *v, size_t n, size_t itemsize)
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{
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void *newptr;
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+ size_t reqalloc, newalloc;
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- newptr = realloc (v->ptr, (n + v->alloc) * itemsize);
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+ reqalloc = v->alloc + n;
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+ if (reqalloc < v->alloc)
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+ return -1; /* overflow */
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+
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+ newalloc = (v->alloc * 3 + 1) / 2;
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+
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+ if (newalloc < reqalloc)
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+ newalloc = reqalloc;
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+
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+ newptr = realloc (v->ptr, newalloc * itemsize);
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if (newptr == NULL)
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return -1;
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v->ptr = newptr;
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- v->alloc += n;
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+ v->alloc = newalloc;
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return 0;
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}
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--
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2.31.1
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