Increase MAX_MEM_PREALLOC_THREAD_COUNT from 16 to 32. This was last
touched in 2017 [1] and, since then, physical machine sizes and VMs
therein have continue to get even bigger, both on average and on the
extremes.
For very large VMs, using 16 threads to preallocate memory can be a
non-trivial bottleneck during VM start-up and migration. Increasing
this limit to 32 threads reduces the time taken for these operations.
Test results from quad socket Intel 8490H (4x 60 cores) show a fairly
linear gain of 50% with the 2x thread count increase.
---------------------------------------------
Idle Guest w/ 2M HugePages | Start-up time
---------------------------------------------
240 vCPU, 7.5TB (16 threads) | 2m41.955s
---------------------------------------------
240 vCPU, 7.5TB (32 threads) | 1m19.404s
---------------------------------------------
[1] 1e356fc14bea ("mem-prealloc: reduce large guest start-up and migration
time.")
Signed-off-by: Jon Kohler <[email protected]>
---
util/oslib-posix.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 3c14b72665..dc001da66d 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -61,7 +61,7 @@
#include "qemu/memalign.h"
#include "qemu/mmap-alloc.h"
-#define MAX_MEM_PREALLOC_THREAD_COUNT 16
+#define MAX_MEM_PREALLOC_THREAD_COUNT 32
struct MemsetThread;
--
2.43.0
