[PATCH 3/3] Stop maximizing the bitmap buffer to reduce the risk of OOM.

[Atsushi Kumagai]

We tried to maximize the bitmap buffer to get the best performance,
but the performance degradation caused by multi-cycle processing
looks very small according to the benchmark on 2TB memory:

  https://lkml.org/lkml/2013/3/26/914

This result means we don't need to make an effort to maximize the
bitmap buffer, it will just increase the risk of OOM.

This patch sets a small fixed value (4MB) as a safety limit,
it may be safer and enough in most cases.

Signed-off-by: Atsushi Kumagai <kumagai-atsushi at mxc.nes.nec.co.jp>
---
 makedumpfile.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/makedumpfile.c b/makedumpfile.c
index b8f1ec4..c23667b 100644
--- a/makedumpfile.c
+++ b/makedumpfile.c
@@ -8946,13 +8946,15 @@ out:
 
 
 /*
- * Choose the lesser value of the two below as the size of cyclic buffer.
- *  - the size enough for storing the 1st/2nd bitmap for the whole of vmcore
- *  - 80% of free memory (as safety limit)
+ * Choose the lesser value of the three below as the size of cyclic buffer.
+ *  - the size enough for storing the 1st or 2nd bitmap for the whole of vmcore
+ *  - 4MB as sufficient value
+ *  - 60% of free memory as safety limit
  */
 int
 calculate_cyclic_buffer_size(void) {
 	unsigned long long limit_size, bitmap_size;
+	const unsigned long long maximum_size = 4 * 1024 * 1024;
 
 	if (info->max_mapnr <= 0) {
 		ERRMSG("Invalid max_mapnr(%llu).\n", info->max_mapnr);
@@ -8960,17 +8962,18 @@ calculate_cyclic_buffer_size(void) {
 	}
 
 	/*
-	 *  We should keep the size of cyclic buffer within 80% of free memory
-	 *  for safety.
+	 *  At least, we should keep the size of cyclic buffer within 60% of
+	 *  free memory for safety.
 	 */
-	limit_size = get_free_memory_size() * 0.8;
+	limit_size = get_free_memory_size() * 0.6;
 	bitmap_size = info->max_mapnr / BITPERBYTE;
 
 	/* if --split was specified cyclic buffer allocated per dump file */
 	if (info->num_dumpfile > 1)
 		bitmap_size /= info->num_dumpfile;
 
-	info->bufsize_cyclic = MIN(limit_size, bitmap_size);
+	/* 4MB will be enought for performance according to benchmarks. */
+	info->bufsize_cyclic = MIN(MIN(limit_size, maximum_size), bitmap_size);
 
 	return TRUE;
 }
-- 
1.9.0