Comparison with Hive-LLAP
In conjunction with the ability to execute multiple TaskAttempts concurrently inside a single ContainerWorker, the support for LLAP I/O makes Hive on MR3 functionally equivalent to Hive-LLAP. Hence Hive on MR3 can serve as a substitute for Hive-LLAP in typical use cases.
Although the two systems are functionally equivalent, Hive on MR3 and Hive-LLAP are fundamentally different in their architecture. In the case of Hive on MR3, a single MR3 DAGAppMaster can execute multiple DAGs concurrently, thus eliminating the need for one DAGAppMaster per DAG as in Hive-LLAP (if HiveServer2 runs in shared session mode). More importantly, the DAGAppMaster has full control over all ContainerWorkers, thus eliminating the potential performance overhead due to lack of knowledge on their internal states. For example, the DAGAppMaster never sends more TaskAttempts than a ContainerWorker can accommodate, which implies that the logic for executing TaskAttempts inside a ContainerWorker is very simple.
In contrast, Hive-LLAP executes multiple DAGs through an interaction between a group of Tez DAGAppMasters, which are effectively owned by HiveServer2, and another group of LLAP daemons. Since DAGAppMasters and LLAP daemons are independent processes (e.g., killing DAGAppMasters does not affect LLAP daemons, and vice versa), the interaction between the two groups is inevitably more complex than in Hive on MR3. In fact, all individual DAGAppMasters themselves are independent processes and compete for the service provided by LLAP daemons, thereby adding another layer of complexity in the architecture.
The difference in the architecture leads to distinct characteristics of the two systems. Below we present the advantage that the simplicity in the design of Hive on MR3 engenders with respect to Hive-LLAP.
1. Higher stability and faster execution
The simplicity in the design of Hive on MR3 yields a noticeable difference in performance, especially in highly concurrent environments. For experimental results, we refer the reader to the article: Performance Evaluation of SQL-on-Hadoop Systems using the TPC-DS Benchmark.
2. Elastic allocation of cluster resources
For Hive-LLAP, the administrator user should decide in advance the number of LLAP daemons and their resource usage after taking into account the plan for running the cluster. Once started, LLAP daemons do not release their resources, which comes with two undesirable consequences:
- When LLAP daemons are idle, their resources cannot be reassigned to busy applications in the same cluster.
- When LLAP daemons are busy, there is no way to exploit idle resources in the cluster.
These problems are particularly relevant to Hive-LLAP because LLAP daemons usually consume very large containers, often the largest containers that Yarn can allocate. In contrast, Hive on MR3 suffers from no such problems because ContainerWorkers are allocated directly from Yarn:
- In order to release idle ContainerWorkers more often, the user only needs to adjust the value for the configuration key
mr3-site.xml. Alternatively the user can just kill idle ContainerWorkers manually.
- When idle resources exist, Hive on MR3 automatically requests more resources on behalf of its ContainerWorkers.
In this way, Hive on MR3 cooperates with existing applications while trying to maximize its own resource usage.
3. Better support for concurrency
In Hive-LLAP, the degree of concurrency is limited by the maximum number of Tez DAGAppMasters that can be created at once by Yarn.
This is because each running query requires a dedicated Tez DAGAppMaster for managing its TaskAttempts.
In practice, the administrator user can impose a hard limit on the number of concurrent queries
by setting the configuration key
hive-site.xml to a suitable value.
In Hive on MR3,
if HiveServer2 runs in shared session mode,
the degree of concurrency is limited only by the amount of memory allocated to a single MR3 DAGAppMaster managing all concurrent queries.
The administrator user may also set the configuration key
mr3-site.xml to specify the maximum number of concurrent queries.
Note that HiveServer2 can also run in individual session mode, in which case the degree of concurrency is limited by the maximum number of MR3 DAGAppMasters like in Hive-LLAP.
In comparison with Hive-LLAP, the use of a single DAGAppMaster brings an advantage to Hive on MR3: in the presence of many concurrent queries, it consumes much less memory for a single MR3 DAGAppMaster than Hive-LLAP consumes for as many Tez DAGAppMasters. Moreover we observe virtually no performance penalty for sharing a DAGAppMaster for many concurrent queries. This is because the computational load on the DAGAppMaster is proportional not to the number of concurrent queries but to the total number of active TaskAttempts, which cannot exceed the limit determined by the total cluster resources.
4. Easier deployment
As it does not require ZooKeeper and Slider, Hive on MR3 is (so much) easier to deploy than Hive-LLAP. For example, in a Hadoop cluster running HDP (Hortonworks Data Platform) 3.0.1, the user should be able to install and run Hive on MR3 just in 10 minutes because Metastore of HDP can be used for Hive on MR3 as well. In essence, Hive on MR3 is just an ordinary application, rather than a special service like Hive-LLAP, from the viewpoint of Yarn.
5. Support for
In principle, Hive on MR3 always allows the user to set the configuration key
hive.server2.enable.doAs to true
because it is just an ordinary application running on top of MR3.
When LLAP I/O is enabled in a secure cluster, however, setting
hive.server2.enable.doAs to true can be a problem
if the LLAP I/O module caches data across many users with different access privileges.
This is also the reason why Hive-LLAP disallows
hive.server2.enable.doAs set to true.
On the other hand, as long as LLAP I/O is disabled,
hive.server2.enable.doAs in Hive on MR3 can be safely set to true.
Hence, for those environments in which 1) LLAP I/O is optional and 2)
hive.server2.enable.doAs should be set to true,
Hive on MR3 is a better choice than the only alternative available, namely Hive on Tez.
For such environments, Hive on MR3 overcomes a well-known limitation of Hive-LLAP.
- By taking advantage of LocalProcess mode in MR3, HiveServer2 can start a DAGAppMaster outside the Hadoop cluster, e.g., on the same machine where HiveServer2 itself runs. This feature is also useful for testing and debugging because the log file for the DAGAppMaster is easily accessible.
- HiveCLI can use LLAP I/O. Note, however, that different instances of HiveCLI do not share the cache for LLAP I/O because each instance maintains its own set of ContainerWorkers.