MDEV-25342: autosize innodb_buffer_pool_chunk_size

The previous default innodb_buffer_pool_chunk_size of 128M
made sense when the innodb buffer pool size was a few GB.

When the pool size is 128GB this means the chunk size is 0.1%
of this. Fine tuning the buffer pool size on such a fine
increment doesn't make practical sense. Also on extremely
large buffer pool systems, initializing on the default 128M can
also take a considerable amount of time.

When large pages are enabled, the chunk size has to be a multiple
of an available large page size or memory allocation without
use can occur.

Previously the default 0 was documented as disabling resizing.
With srv_buf_pool_chunk_unit > 0 assertions in the code and the
minimium value set, I doubt this was ever the case.

As such the autosizing (based on default 0) takes place as follows:
* a 64th of the innodb_buffer_pool_size
* if large pages, this is rounded down the the nearest multiple
  of the large page size.
* If less than 1MB, set to 1MB.

This does mean the new default innodb_buffer_pool_chunk size is
2MB, derived form the above formular with 128MB as the buffer pool
size.

The innodb_buffer_pool_chunk_size is changed to a size_t for
better compatiblity with the memory allocations which use size_t.
The previous upper limit is changed to the maxium of a size_t. The
maximium value used is the buffer pool size anyway.

Getting this default value of the chunk size to a more practical
size facilitates further development of more automated resizing
without significant overhead or memory fragmentation.

innodb_buffer_pool_resize test adjusted based on 1M default
chunk size thanks Wlad.

1 files changed, 22 insertions, 0 deletions

diff --git a/mysys/my_largepage.c b/mysys/my_largepage.c
index 0fdc4e17a26..9cc1e19772b 100644
--- a/mysys/my_largepage.c
+++ b/mysys/my_largepage.c
@@ -251,6 +251,28 @@ int my_init_large_pages(my_bool super_large_pages)
 }
 
 
+/**
+   Large page size helper.
+   This rounds down, if needed, the size parameter to the largest
+   multiple of an available large page size on the system.
+*/
+void my_large_page_truncate(size_t *size)
+{
+  if (my_use_large_pages)
+  {
+    size_t large_page_size= 0;
+#ifdef _WIN32
+    large_page_size= my_large_page_size;
+#elif defined(HAVE_MMAP)
+    int page_i= 0;
+    large_page_size= my_next_large_page_size(*size, &page_i);
+#endif
+    if (large_page_size > 0)
+      *size-= *size % large_page_size;
+  }
+}
+
+
 #if defined(HAVE_MMAP) && !defined(_WIN32)
 /* Solaris for example has only MAP_ANON, FreeBSD has MAP_ANONYMOUS and
 MAP_ANON but MAP_ANONYMOUS is marked "for compatibility" */