[Qemu-devel] [PATCH] numa: equally distribute memory on nodes

[Laurent Vivier]

When there is more nodes than memory available to put the minimum
allowed memory by node, all the memory is put on the last node.

This is because we put (ram_size / nb_numa_nodes) &
~((1 << mc->numa_mem_align_shift) - 1); on each node, and in this
case the value is 0. This is particularly true with pseries,
as the memory must be aligned to 256MB.

To avoid this problem, this patch uses an error diffusion algorithm [1]
to distribute equally the memory on nodes.

Example:

qemu-system-ppc64 -S -nographic  -nodefaults -monitor stdio -m 1G -smp 8 \
                  -numa node -numa node -numa node \
                  -numa node -numa node -numa node

Before:

(qemu) info numa
6 nodes
node 0 cpus: 0 6
node 0 size: 0 MB
node 1 cpus: 1 7
node 1 size: 0 MB
node 2 cpus: 2
node 2 size: 0 MB
node 3 cpus: 3
node 3 size: 0 MB
node 4 cpus: 4
node 4 size: 0 MB
node 5 cpus: 5
node 5 size: 1024 MB

After:
(qemu) info numa
6 nodes
node 0 cpus: 0 6
node 0 size: 0 MB
node 1 cpus: 1 7
node 1 size: 256 MB
node 2 cpus: 2
node 2 size: 0 MB
node 3 cpus: 3
node 3 size: 256 MB
node 4 cpus: 4
node 4 size: 256 MB
node 5 cpus: 5
node 5 size: 256 MB

[1] https://en.wikipedia.org/wiki/Error_diffusion

Signed-off-by: Laurent Vivier <address@hidden>
---
 numa.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/numa.c b/numa.c
index 6fc2393..bcf1c54 100644
--- a/numa.c
+++ b/numa.c
@@ -336,15 +336,19 @@ void parse_numa_opts(MachineClass *mc)
             }
         }
         if (i == nb_numa_nodes) {
-            uint64_t usedmem = 0;
+            uint64_t usedmem = 0, node_mem;
+            uint64_t granularity = ram_size / nb_numa_nodes;
+            uint64_t propagate = 0;
 
             /* Align each node according to the alignment
              * requirements of the machine class
              */
             for (i = 0; i < nb_numa_nodes - 1; i++) {
-                numa_info[i].node_mem = (ram_size / nb_numa_nodes) &
+                node_mem = (granularity + propagate) &
                                         ~((1 << mc->numa_mem_align_shift) - 1);
-                usedmem += numa_info[i].node_mem;
+                propagate = granularity + propagate - node_mem;
+                numa_info[i].node_mem = node_mem;
+                usedmem += node_mem;
             }
             numa_info[i].node_mem = ram_size - usedmem;
         }
-- 
2.9.3