Chained item cannot be larger than 4MB (pmem#85)

Summary: Pull Request resolved: facebook#85

Reviewed By: sathyaphoenix

Differential Revision: D32806006

Pulled By: leozzx

fbshipit-source-id: b973a5bd5a3e1b7a566efb6887de21c0f8f756c4

Author: xiedeyantu

cachelib/allocator/CacheItem.h

@@ -472,8 +472,8 @@ class CACHELIB_PACKED_ATTR CacheItem {
 // |  a |       | y |      |
 // |  t |       | l |      |
 // |  i |       | o |      |
-// |  o |       | d |      |
-// |  n |       |   |      |
+// |  o |       | a |      |
+// |  n |       | d |      |
 // | --------------------- |
 template <typename CacheTrait>
 class CACHELIB_PACKED_ATTR CacheChainedItem : public CacheItem<CacheTrait> {

website/docs/Cache_Library_User_Guides/Write_data_to_cache.md

@@ -78,9 +78,11 @@ auto parent_item_handle = cache->allocate(pool_id, "parent", size);
 
 if (parent_item_handle) {
   // Call allocateChainedItem() to allocate memory for 3 chained items.
-  auto chained_item_handle_1 = cache->allocateChainedItem(parent_item_handle, 2 * size);
-  auto chained_item_handle_2 = cache->allocateChainedItem(parent_item_handle, 4 * size);
-  auto chained_item_handle_3 = cache->allocateChainedItem(parent_item_handle, 6 * size);
+  // 4 * size is invalid, Because there are other costs, see getRequiredSize().
+  // But you can create more chained items.
+  auto chained_item_handle_1 = cache->allocateChainedItem(parent_item_handle, 1 * size);
+  auto chained_item_handle_2 = cache->allocateChainedItem(parent_item_handle, 2 * size);
+  auto chained_item_handle_3 = cache->allocateChainedItem(parent_item_handle, 3 * size);
 }
 ```
 

website/docs/Cache_Library_User_Guides/chained_items.md

@@ -97,7 +97,7 @@ std::unique_ptr<LargeUserData> userData = getLargeUserData();
 size_t userDataSize = sizeof(LargeUserData) + sizeof(int) * userData->length;
 
 // For simplicity, we'll split the user data into 1MB chunks
-size_t numChunks = userDataSize / (1024 * 1024 * 1024);
+size_t numChunks = userDataSize / (1024 * 1024);
 
 struct CustomParentItem {
   size_t numChunks;