[GitHub] [incubator-doris] xinyiZzz commented on a diff in pull request #9250: [Enhancement][Vectorized] Improve hash table build efficiency

[GitBox <gi...@apache.org>]

xinyiZzz commented on code in PR #9250:
URL: https://github.com/apache/incubator-doris/pull/9250#discussion_r859755912


##########
be/src/vec/common/allocator.h:
##########
@@ -146,8 +146,13 @@ class Allocator {
                                                           std::to_string(old_size) + " to " +
                                                           std::to_string(new_size) + ".",
                                                   doris::TStatusCode::VEC_CANNOT_MREMAP);
-
             /// No need for zero-fill, because mmap guarantees it.
+
+            if constexpr (mmap_populate)
+                // MAP_POPULATE seems have no effect for mremap as for mmap,
+                // Zero-fill enlarged memory range explicitly to pre-fault the pages
+                if (new_size > old_size)
+                    memset(reinterpret_cast<char*>(buf) + old_size, 0, new_size - old_size);

Review Comment:
   Why manual zero-fill is needed here,
   
   CK's comment mentions that mmap is automatically zero-filled. The manual also seems to say so: https://linux.die.net/man/2/mmap



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