[Qemu-devel] [PATCH 2/2] find_ram_offset: Align ram_addr_t allocation on long boundaries

[Paolo Bonzini <pbonzini@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

The dirty bitmaps are built from 'long's and there is fast-path code
for synchronising the case where the RAMBlock is aligned to the start
of a long boundary.  Align the allocation to this boundary
to cause the fast path to be used.

Offsets before change:
11398@1515169675.018566:find_ram_offset size: 0x1e0000 @ 0x8000000
11398@1515169675.020064:find_ram_offset size: 0x20000 @ 0x81e0000
11398@1515169675.020244:find_ram_offset size: 0x20000 @ 0x8200000
11398@1515169675.024343:find_ram_offset size: 0x1000000 @ 0x8220000
11398@1515169675.025154:find_ram_offset size: 0x10000 @ 0x9220000
11398@1515169675.027682:find_ram_offset size: 0x40000 @ 0x9230000
11398@1515169675.032921:find_ram_offset size: 0x200000 @ 0x9270000
11398@1515169675.033307:find_ram_offset size: 0x1000 @ 0x9470000
11398@1515169675.033601:find_ram_offset size: 0x1000 @ 0x9471000

after change:
10923@1515169108.818245:find_ram_offset size: 0x1e0000 @ 0x8000000
10923@1515169108.819410:find_ram_offset size: 0x20000 @ 0x8200000
10923@1515169108.819587:find_ram_offset size: 0x20000 @ 0x8240000
10923@1515169108.823708:find_ram_offset size: 0x1000000 @ 0x8280000
10923@1515169108.824503:find_ram_offset size: 0x10000 @ 0x9280000
10923@1515169108.827093:find_ram_offset size: 0x40000 @ 0x92c0000
10923@1515169108.833045:find_ram_offset size: 0x200000 @ 0x9300000
10923@1515169108.833504:find_ram_offset size: 0x1000 @ 0x9500000
10923@1515169108.833787:find_ram_offset size: 0x1000 @ 0x9540000

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180105170138.23357-3-dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 exec.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/exec.c b/exec.c
index 829f9d8d1a..c543b54094 100644
--- a/exec.c
+++ b/exec.c
@@ -1679,7 +1679,11 @@ static ram_addr_t find_ram_offset(ram_addr_t size)
     RAMBLOCK_FOREACH(block) {
         ram_addr_t candidate, next = RAM_ADDR_MAX;
 
+        /* Align blocks to start on a 'long' in the bitmap
+         * which makes the bitmap sync'ing take the fast path.
+         */
         candidate = block->offset + block->max_length;
+        candidate = ROUND_UP(candidate, BITS_PER_LONG << TARGET_PAGE_BITS);
 
         /* Search for the closest following block
          * and find the gap.
-- 
2.14.3