Don't use MADV_RANDOM

In addition to the explicit documented behavior in posix_madvise(2) this
call since Linux 6.4 also causes the kernel to aggressively free pages
from the page cache by short circuiting the LRU second chance mechanism.
The result is compaction events that took 900ms now take up to 20s and
a system which generally operated with near zero major page faults sees
600 or more major faults per second during compaction events.

We've tested this change in older kernels and observed no negative impact
in typical cloud instances.

Fixes #939

Author: sdodson

bolt_unix.go

@@ -3,7 +3,6 @@
 package bbolt
 
 import (
-	"fmt"
 	"syscall"
 	"time"
 	"unsafe"
@@ -58,13 +57,6 @@ func mmap(db *DB, sz int) error {
 		return err
 	}
 
-	// Advise the kernel that the mmap is accessed randomly.
-	err = unix.Madvise(b, syscall.MADV_RANDOM)
-	if err != nil && err != syscall.ENOSYS {
-		// Ignore not implemented error in kernel because it still works.
-		return fmt.Errorf("madvise: %s", err)
-	}
-
 	// Save the original byte slice and convert to a byte array pointer.
 	db.dataref = b
 	db.data = (*[maxMapSize]byte)(unsafe.Pointer(&b[0]))