tune ancient append vec size to 128M (#34067)

* tune ancient append vec size to 130M

* fix a test and get rid of the assert since it is covered in the test

* use 128M

* assert max append vec size for ancient append vec

---------

Co-authored-by: HaoranYi <haoran.yi@solana.com>

accounts-db/src/ancient_append_vecs.rs

@@ -942,13 +942,26 @@ impl<'a> AccountsToStore<'a> {
 }
 
 /// capacity of an ancient append vec
-pub fn get_ancient_append_vec_capacity() -> u64 {
+#[allow(clippy::assertions_on_constants, dead_code)]
+pub const fn get_ancient_append_vec_capacity() -> u64 {
+    // There is a trade-off for selecting the ancient append vec size. Smaller non-ancient append vec are getting
+    // combined into large ancient append vec. Too small size of ancient append vec will result in too many ancient append vec
+    // memory mapped files. Too big size will make it difficult to clean and shrink them. Hence, we choose approximately
+    // 128MB for the ancient append vec size.
+    const RESULT: u64 = 128 * 1024 * 1024;
+
     use crate::append_vec::MAXIMUM_APPEND_VEC_FILE_SIZE;
-    // smaller than max by a bit just in case
-    // some functions add slop on allocation
-    // The bigger an append vec is, the more unwieldy it becomes to shrink, create, write.
-    // 1/10 of max is a reasonable size in practice.
-    MAXIMUM_APPEND_VEC_FILE_SIZE / 10 - 2048
+    const _: () = assert!(
+        RESULT < MAXIMUM_APPEND_VEC_FILE_SIZE,
+        "ancient append vec size should be less than the maximum append vec size"
+    );
+    const PAGE_SIZE: u64 = 4 * 1024;
+    const _: () = assert!(
+        RESULT % PAGE_SIZE == 0,
+        "ancient append vec size should be a multiple of PAGE_SIZE"
+    );
+
+    RESULT
 }
 
 /// is this a max-size append vec designed to be used as an ancient append vec?
@@ -2078,10 +2091,7 @@ pub mod tests {
 
     #[test]
     fn test_get_ancient_append_vec_capacity() {
-        assert_eq!(
-            get_ancient_append_vec_capacity(),
-            crate::append_vec::MAXIMUM_APPEND_VEC_FILE_SIZE / 10 - 2048
-        );
+        assert_eq!(get_ancient_append_vec_capacity(), 128 * 1024 * 1024);
     }
 
     #[test]