Benchmarks, Redux (part 8): BSBM Explore and Update

We will here look at the Explore and Update scenario of BSBM. This presents us with a novel problem as the specification does not address any aspect of ACID.

A transaction benchmark ought to have something to say about this. The SPARUL (also known as SPARQL/Update) language does not say anything about transactionality, but I suppose it is in the spirit of the SPARUL protocol to promise atomicity and durability.

We begin by running Virtuoso 7 Single, with Single User and 16 User, each at scales of 100 Mt, 200 Mt, and 1000 Mt. The transactionality is default, meaning SERIALIZABLE isolation between INSERTs and DELETEs, and READ COMMITTED isolation between READ and any UPDATE transaction. (Figures for Virtuoso 6 will also be presented here in the near future, as they are the currently shipping production versions.)

Virtuoso 7 Single, Full ACID (QMpH, query mixes per hour)
Scale	Single User	16 User
100 Mt	9,969	65,537
200 Mt	8,646	40,527
1000 Mt	5,512	17,293

Virtuoso 6 Cluster, Full ACID (QMpH, query mixes per hour)
Scale	Single User	16 User
100 Mt	5604.520	34079.019
1000 Mt	2866.616	10028.325

Virtuoso 6 Single, Full ACID (QMpH, query mixes per hour)
Scale	Single User	16 User
100 Mt	7,152	21,065
200 Mt	5,862	16,895
1000 Mt	1,542	4,548

Each run is preceded by a warm-up of 500 or 300 mixes (the exact number is not material), resulting in a warm cache; see previous post on read-ahead for details. All runs do 1000 Explore and Update mixes. The initial database is in the state following the Explore only runs.

The results are in line with the Explore results. There is a fair amount of variability between consecutive runs; the 16 User run at 1000 Mt varies between 14K and 19K QMpH depending on the measurement. The smaller runs exhibit less variability.

In the following we will look at transactions and at how the definition of the workload and reporting could be made complete.

Full ACID means serializable semantic of concurrent insert and delete of the same quad. Non-transactional means that on concurrent insert and delete of overlapping sets of quads the result is undefined. Further if one logged such "transactions," the replay would give serialization although the initial execution did not, hence further confusing the issue. Considering the hypothetical use case of an e-commerce information portal, there is little chance of deletes and inserts actually needing serialization. An insert-only workload does not need serializability because an insert cannot fail. If the data already exists the insert does nothing, if the quad does not previously exist it is created. The same applies to deletes alone. If a delete and insert overlap, serialization would be needed but the semantics implicit in the use case make this improbable.

Read-only transactions (i.e., the Explore mix in the Explore and Update scenario) will be run as READ COMMITTED. These do not see uncommitted data and never block for lock wait. The reads may not be repeatable.

Our first point of call is to determine the cost of ACID. We run 1000 mixes of Explore and Update at 1000 Mt. The throughput is 19214 after a warm-up of 500 mixes. This is pretty good in comparison with the diverse read-only results at this scale.

We look at the pertinent statistics:

SELECT TOP 5 * FROM sys_l_stat ORDER BY waits DESC;

KEY_TABLE         INDEX_NAME       LOCKS   WAITS   WAIT_PCT   DEADLOCKS   LOCK_ESC   WAIT_MSECS
===============   =============   ======   =====   ========   =========   ========   ==========
DB.DBA.RDF_QUAD   RDF_QUAD_POGS   179205     934          0           0          0        35164
DB.DBA.RDF_IRI    RDF_IRI          20752     217          1           0          0        16445
DB.DBA.RDF_QUAD   RDF_QUAD_SP       9244       3          0           0          0          235

We see 934 waits with a total duration of 35 seconds on the index with the most contention. The run was 187 seconds, real time. The lock wait time is not real time since this is the total elapsed wait time summed over all threads. The lock wait frequency is a little over one per query mix, meaning a little over one per five locking transactions.

We note that we do not get deadlocks since all inserts and deletes are in ascending key order due to vectoring. This guarantees the absence of deadlocks for single insert transactions, as long as the transaction stays within the vector size. This is always the case since the inserts are a few hundred triples at the maximum. The waits concentrate on POGS, because this is a bitmap index where the locking resolution is less than a row, and the values do not correlate with insert order. The locking behavior could be better with the column store, where we would have row level locking also for this index. This is to be seen. The column store would otherwise tend to have higher cost per random insert.

Considering these results it does not seem crucial to "drop ACID," though doing so would save some time. We will now run measurements for all scales with 16 Users and ACID.

Let us now see what the benchmark writes:

SELECT TOP 10 * FROM sys_d_stat ORDER BY n_dirty DESC;

KEY_TABLE                     INDEX_NAME                       TOUCHES     READS   READ_PCT   N_DIRTY   N_BUFFERS
===========================   ============================   =========   =======   ========   =======   =========
DB.DBA.RDF_QUAD               RDF_QUAD_POGS                  763846891    237436          0     58040      228606
DB.DBA.RDF_QUAD               RDF_QUAD                       213282706   1991836          0     30226     1940280
DB.DBA.RDF_OBJ                RO_VAL                             15474     17837        115     13438       17431
DB.DBA.RO_START               RO_START                           10573     11195        105     10228       11227
DB.DBA.RDF_IRI                RDF_IRI                            61902    125711        203      7705      121300
DB.DBA.RDF_OBJ                RDF_OBJ                         23809053   3205963         13       636     3072517
DB.DBA.RDF_IRI                DB_DBA_RDF_IRI_UNQC_RI_ID        3237687    504486         15       340      488797
DB.DBA.RDF_QUAD               RDF_QUAD_SP                        89995     70446         78        99       68340
DB.DBA.RDF_QUAD               RDF_QUAD_OP                        19440     47541        244        66       45583
DB.DBA.VTLOG_DB_DBA_RDF_OBJ   VTLOG_DB_DBA_RDF_OBJ                3014         1          0        11          11
DB.DBA.RDF_QUAD               RDF_QUAD_GS                         1261       801         63        10         751
DB.DBA.RDF_PREFIX             RDF_PREFIX                            14       168       1120         1         153
DB.DBA.RDF_PREFIX             DB_DBA_RDF_PREFIX_UNQC_RP_ID        1807       200         11         1         200

The most dirty pages are on the POGS index, which is reasonable; values are spread out at random. After this we have the PSOG index, likely because of random deletes. New IRIs tend to get consecutive numbers and do not make many dirty pages. Literals come next, with the index from leading string or hash of the literal to id leading, as one would expect, again because of values being distributed at random. After this come IRIs. The distribution of updates is generally as one would expect.

* * *

Going back to BSBM, at least the following aspects of the benchmark have to be further specified:

Disclosure of ACID properties. If the benchmark required full ACID many would not run this at all. Besides full ACID is not necessarily an absolute requirement based on the hypothetical usage scenario of the benchmark. However, when publishing numbers the guarantees that go with the numbers must be made explicit. This includes logging, checkpoint frequency or equivalent etc.
Steady state. The working set of the Update mix is different from that of the Explore mixes. This touches more indices than Explore. The Explore warm-up is in part good but does not represent steady state.
Checkpoint and sustained throughput. Benchmarks involving update generally have rules for checkpointing the state and for sustained throughput. In specific, the throughput of an update benchmark cannot rely on never flushing to persistent storage. Even bulk load must be timed with a checkpoint guaranteeing durability at the end. A steady update stream should be timed with a test interval of sufficient length involving a few checkpoints; for example, a minimum duration of 30 minutes with no less than 3 completed checkpoints in the interval with at least 9 minutes between the end of one and the start of the next. Not all DBMSs work with logs and checkpoints, but if an alternate scheme is used then this needs to be described.
Memory and warm-up issues.We have seen the test data generator run out of memory when trying to generate update streams of meaningful length. Also the test driver should allow running updates in timed and non-timed mode (warm-up).

With an update benchmark, many more things need to be defined, and the set-up becomes more system specific, than with a read-only workload. We will address these shortcomings in the measurement rules proposal to come. Especially with update workloads, the vendors need to provide tuning expertise; however, this will not happen if the benchmark does not properly set the expectations. If benchmarks serve as a catalyst for clearly defining how things are to be set up, then they will have served the end user.