Accumulo scans operate by repeatedly fetching batches of data from a tablet server. On the tablet server side, a thread pool fetches batches. In Java threads pools are called executors. By default, a single executor per tablet server handles all scans in FIFO order. For some workloads, the single FIFO executor is suboptimal. For example, consider many unimportant scans reading lots of data mixed with a few important scans reading small amounts of data. The long scans noticeably increase the latency of the short scans. Accumulo offers two mechanisms to help improve situations like this: multiple scan executors and per executor prioritizers. Additional scan executors can give tables dedicated resources. For each scan executor, an optional prioritizer can reorder queued work.

Configuring and using Scan Executors

By default, Accumulo sets tserver.scan.executors.default.threads=16 which creates the default scan executor. To configure additional scan executors, chose a unique name and configure tserver.scan.executors.*. Setting the following causes each tablet server to create a scan executor with the specified threads.

tserver.scan.executors.<name>.threads=<number>

Optionally, some of the following can be set. The priority setting determines thread priority. The prioritizer settings specifies a class that orders pending work.

tserver.scan.executors.<name>.priority=<number 1 to 10>
tserver.scan.executors.<name>.prioritizer=<class name>
tserver.scan.executors.<name>.prioritizer.opts.<key>=<value>

After creating an executor, configure table.scan.dispatcher to use it. A dispatcher is Java subclass of ScanDispatcher that decides which scan executor should service a table. Set the following table property to configure a dispatcher.

table.scan.dispatcher=<class name>

Scan dispatcher options can be set with properties like the following.

table.scan.dispatcher.opts.<key>=<value>

The default value for table.scan.dispatcher is SimpleScanDispatcher. SimpleScanDispatcher supports an executor option for choosing a scan executor. If this option is not set, then SimpleScanDispatcher will dispatch to the scan executor named default.

To tie everything together, consider the following use case.

Create tables named LOW1 and LOW2 using a scan executor with a single thread.
Create a table named HIGH with a dedicated scan executor with 8 threads.
Create tables named NORM1 and NORM2 using the default scan executor.
Set the default executor to 4 threads.

The following shell commands implement this use case.

createtable LOW1
createtable LOW2
createtable HIGH
createtable NORM1
createtable NORM2
config -s tserver.scan.executors.default.threads=4
config -s tserver.scan.executors.low.threads=1
config -s tserver.scan.executors.high.threads=8

Tablet servers should be restarted after configuring scan executors, then tables can be configured.

config -t LOW1 -s table.scan.dispatcher=org.apache.accumulo.core.spi.scan.SimpleScanDispatcher
config -t LOW1 -s table.scan.dispatcher.opts.executor=low
config -t LOW2 -s table.scan.dispatcher=org.apache.accumulo.core.spi.scan.SimpleScanDispatcher
config -t LOW2 -s table.scan.dispatcher.opts.executor=low
config -t HIGH -s table.scan.dispatcher=org.apache.accumulo.core.spi.scan.SimpleScanDispatcher
config -t HIGH -s table.scan.dispatcher.opts.executor=high

While not necessary because it’s the default, it is safer to also set table.scan.dispatcher=org.apache.accumulo.core.spi.scan.SimpleScanDispatcher for each table. This ensures things work as expected in the case where table.scan.dispatcher was set at the system or namespace level.

Configuring and using Scan Prioritizers.

When all scan executor threads are busy, incoming work is queued. By default, this queue has a FIFO order. A ScanPrioritizer can be configured to reorder the queue. Accumulo ships with the IdleRatioScanPrioritizer which orders the queue by the ratio of run time to idle time. For example, a scan with a run time of 50ms and an idle time of 200ms would have a ratio of .25. If .25 were the lowest ratio on the queue, then it would be the next in line. The following configures the IdleRatioScanPrioritizer for the default scan executor.

tserver.scan.executors.default.prioritizer=org.apache.accumulo.core.spi.scan.IdleRatioScanPrioritizer

Using the IdleRatioScanPrioritizer in a test with 50 long running scans and 5 threads repeatedly doing small random lookups made a significant difference. In this test the average lookup time for the 5 threads went from 250ms to 5 ms.

Providing hints from the client side.

Scanners can provide hints to ScanDispatchers and ScanPriotizers by calling setExecutionHints on the Scanner. What, if anything, is done with these hints depends on what is configured for the table and system. Accumulo’s default configuration ignores hints. The following shell commands make it possible to choose an executor and set priorities from a scanner for the table tex.

config -s tserver.scan.executors.special.threads=8
config -s tserver.scan.executors.special.prioritizer=org.apache.accumulo.core.spi.scan.HintScanPrioritizer
config -s tserver.scan.executors.special.prioritizer.opts.priority.alpha=1
config -s tserver.scan.executors.special.prioritizer.opts.priority.gamma=3
createtable tex
config -t tex -s table.scan.dispatcher=org.apache.accumulo.core.spi.scan.SimpleScanDispatcher
config -t tex -s table.scan.dispatcher.opts.executor.alpha=special
config -t tex -s table.scan.dispatcher.opts.executor.gamma=special

The HintScanPrioritizer honors hints of the form priority=<integer> or scan_type=<type> to prioritize scans, with lower integers resulting in a higher priority. When a hint specifies a scan type it is mapped to a priority based on the prioritizer configuration.

The SimpleScanDispatcher, which is the default dispatcher, supports executor.<type>=<executor> options. When a scanner sets a hint of the form scan_type=<type> it will use the executor configured for that type.

After restarting tservers, the following command will start a scan that uses the executor special with a priority of 3. The scan dispatcher maps the scan type gamma to the executor special. The prioritizer maps the scan type gamma to a priority of 3.

scan -t tex --execution-hints scan_type=gamma

The following command will start a scan that uses the executor special with a priority of 1.

scan -t tex --execution-hints scan_type=alpha

Execution Hints can also be used to influence how the block caches are used for a scan. The following configuration would modify the gamma executor to use blocks in the cache if they are already cached, but would never load missing blocks into the cache.

config -t tex -s table.scan.dispatcher.opts.cacheUsage.gamma=opportunistic

Other valid values are disabled which does not use data in the block caches, enabled which uses the block cache as it normally would and table which enables the block cache for the scan if it’s enabled on the table.

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