Merge pull request #3082 from pnorbert/bufferchunksize-default-128mb

Set BP5's BufferChunkSize to 128MB by default.

docs/user_guide/source/engines/bp5.rst

@@ -78,7 +78,7 @@ This engine allows the user to fine tune the buffering operations through the fo
 
    #. **BufferVType**: *chunk* or *malloc*, default is chunking. Chunking maintains the buffer as a list of memory blocks, either ADIOS-owned for sync-ed Puts and small Puts, and user-owned pointers of deferred Puts. Malloc maintains a single memory block and extends it (reallocates) whenever more data is buffered. Chunking incurs extra cost in I/O by having to write data in chunks (multiple write system calls), which can be helped by increasing *BufferChunkSize* and *MinDeferredSize*. Malloc incurs extra cost by reallocating memory whenever more data is buffered (by Put()), which can be helped by increasing *InitialBufferSize*. 
 
-   #. **BufferChunkSize**: (for *chunk* buffer type) The size of each memory buffer chunk, default is 16MB but it is worth increasing up to 2GB if possible for maximum write performance.
+   #. **BufferChunkSize**: (for *chunk* buffer type) The size of each memory buffer chunk, default is 128MB but it is worth increasing up to 2GB if possible for maximum write performance.
 
    #. **MinDeferredSize**: (for *chunk* buffer type) Small user variables are always buffered, default is 4MB. 
 
@@ -134,7 +134,7 @@ This engine allows the user to fine tune the buffering operations through the fo
  StripeSize                     integer+units         **4KB**
  MaxShmSize                     integer+units         **4294762496**
  BufferVType                    string                **chunk**, malloc
- BufferChunkSize                integer+units         **16MB**, worth increasing up to min(2GB, datasize/process/step)
+ BufferChunkSize                integer+units         **128MB**, worth increasing up to min(2GB, datasize/process/step)
  MinDeferredSize                integer+units         **4MB**
  InitialBufferSize              float+units >= 16Kb   **16Kb**, 10Mb, 0.5Gb
  GrowthFactor                   float > 1             **1.05**, 1.01, 1.5, 2

source/adios2/common/ADIOSTypes.h

@@ -248,8 +248,8 @@ constexpr uint64_t DefaultMaxBufferSize = MaxSizeT - 1;
 constexpr float DefaultBufferGrowthFactor = 1.05f;
 
 /** default Buffer Chunk Size
- *  16Mb */
-constexpr uint64_t DefaultBufferChunkSize = 16 * 1024 * 1024;
+ *  128Mb */
+constexpr uint64_t DefaultBufferChunkSize = 128 * 1024 * 1024;
 
 /** default minimum size not copying deferred writes
  *  4Mb */