Hi Vlad, Would be great to get insight from the original authors. Here ismy two cents as a late comer who made extensive use of the classes in question.
Many of your questions are at the implementation level. It is worth looking at the question from two other perspectives: history and design. Historically, Drill adopted Netty for networking, and wisely looked for ways of using the same buffers for both network transfer and internal operations to avoid copies. Some overview is in [1]. In this view, a Drill vector is a network buffer. Network buffers use the ByteBuffer protocol to serialize binary values. DrillBuf follows that model for the most part. Because a ByteBuffer is a low-level abstraction over a buffer, each operation must perform bounds checks to ensure safe operation. DrillBuf provides the ability to present a "view" of a slice of a larger underlying buffer. For example, when reading data from a spill file, all data for all internal vectors is read into a single buffer. For a nullable VarChar, for example, the buffer contains the bit vectors, the offset vectors and the data vectors. The value vectors point to DrillBufs which point to a slice of the underlying buffers. It is this layout (there are at least three different layouts) that makes our "record batch sizer" so complex: the size of memory used is NOT the sum of the DrillBufs. Drill is a columnar system. So, the team introduced a typed "vector" abstraction.Value vectors provide an abstraction that sweeps away the ByteBuffer heritage and replaces it with a strongly typed, accessor/mutator structure that works in terms of Drill data types and record counts. Vectors also understand the relationship between bit vectors and data vectors, between offset vectors and data vectors, and so on. Your question implies a desire to think about the future direction. Two things to say. First, vectors themselves do not provide sufficient abstraction for the needs of operators. As a result, operators become very complex, we must generate large amounts of boiler-plate code, and we fix the same bugs over and over. These issues are discussed at length in [2]. This is the motivation for the result set reader and loader. The row set abstractions encapsulate not just knowledge of a vector, but of the entire batch. As a result, these abstractions know the number of records, know the vector and batch size targets, and track vectors as they fill. One key result is that these abstractions ensure that data is read or written within the bounds of each buffer, eliminating the need for bounds checks on every access. The other consideration is memory management. Drill has a very complex, but surprisingly robust, memory management system. However, it is based on a "malloc" model of memory with operators negotiating among themselves (via the OUT_OF_MEMORY iterator status) about who needs memory and who should release it. [2] discusses the limitations of this system. As a result, we've been moving to a budget-based system in which each fragment and operator is given a budget based on total available memory, and operators use spilling to stay within the budget. Memory fragmentation is a classic problem in malloc-based systems which strive to operate at high memory utilization rates and which do not include memory compaction. Drill is such a system. So, if this issue ever prevents Drill from achieving maximum performance, we can consider the classic system used by databases to solve this problem: fixed-size memory blocks. If we were to move to fixed-size buffers, we'd want the row set and vector abstractions to remain unchanged. We'd only want to replace DrillBuf with a new block-based abstraction, perhaps with chaining (a vector may consist of a chain of, say, 1 MB blocks.) The buffer slicing mechanism would become unnecessary, as would the existing malloc-based allocator. Instead, data would be read, written and held in buffers allocated from and returned to a buffer pool. We may or may not ever make such a change. But, by considering this possibility, we readily see that DrillBuf should be an implementation detail of the higher-level abstractions and that operators should only use those higher-level abstractions because doing so isolates operators from the details of memory layout. This argument applies even more so to the abstractions below DrillBuf: UDLE, Netty ByteBuf, ledgers and so on. Said another way, even with the current system, we should be free to improve DrillBuf on down with no impact to operator code because vectors and the row set abstractions should be the only clients of DrillBuf. In short, by understanding the history of the code, and agreeing upon the right design abstractions, we can then make informed decisions about how best to improve our low-level abstractions, including DrillBuf. Thanks, - Paul [1] http://drill.apache.org/docs/value-vectors/ [2] https://github.com/paul-rogers/drill/wiki/Batch-Handling-Upgrades On Wednesday, April 4, 2018, 10:34:18 AM PDT, Vlad Rozov <vro...@apache.org> wrote: I have several questions and concerns regarding DrillBuf usage, design and implementation. There is a limited documentation available for the subject (Java doc, https://github.com/apache/drill/blob/master/exec/memory/base/src/main/java/org/apache/drill/exec/memory/README.md and https://github.com/paul-rogers/drill/wiki/Memory-Management) and I hope that a few members of the community may have more information. What are the design goals behind DrillBuf? It seems like it is supposed to be Drill access gate for direct byte buffers. How is it different (for that goal) from UnsafeDirectLittleEndian? Both use wrapper/delegation pattern, with DrillBuf delegating to UnsafeDirectLittleEndian (not always) and UnsafeDirectLittleEndian delegating to ByteBuf it wraps. Is it necessary to have both? Are there any out of the box netty classes that already provide required functionality? I guess that answer to the last question was "no" back when DrillBuf and UnsafeDirectLittleEndian were introduced into Drill. Is it still "no" for the latest netty release? What extra functionality DrillBuf (and UnsafeDirectLittleEndian) provides on top of existing netty classes? As far as I can see from the source code, DrillBuf changes validation (boundary and reference count checks) mechanism, making it optional (compared to always enabled boundary checks inside netty) for get/set Byte/Char/Short/Long/Float/Double. Is this a proper place to make validation optional or the validation (or portion of the validation) must be always on or off (there are different opinions, see https://issues.apache.org/jira/browse/DRILL-6004, https://issues.apache.org/jira/browse/DRILL-6202, https://github.com/apache/drill/pull/1060 and https://github.com/apache/drill/pull/1144)? Are there any performance benchmark that justify or explain such behavior (if such benchmark does not exist, are there any volunteer to do the benchmark)? My experience is that the reference count check is significantly more expensive compared to boundary checking and boundary checking adds tens of percent to direct memory read when reading just a few bytes, so my vote is to keep validation as optional with the ability to enable it for debug purposes at run-time. What is the reason the same approach do not apply to get/set Bytes and those methods are delegated to UnsafeDirectLittleEndian that delegates it further? Why DrillBuf reverses how AbstractByteBuf calls _get from get (and _set from set), making _get to call get (_set to call set)? Why not to follow a base class design patter? Another question is usage of netty "io.netty.buffer" package for Drill classes. Is this absolutely necessary? I don't think that netty developers expect this and support semantic version compatibility for package private classes/members. Thank you, Vlad
