Version 2.7.0
Cache and eviction tuning

Cache size

The size of the cache is the single most important tuning knob for a WiredTiger application. Ideally the cache should be configured to be large enough to hold an application's working set.

The cache size for the database is normally configured by setting the cache_size configuration string when calling the wiredtiger_open function. The cache size can be adjusted after the open call with WT_CONNECTION::reconfigure.

An example of setting a cache size to 500MB:

if ((ret = wiredtiger_open(home, NULL,
"create,cache_size=500M", &conn)) != 0)
fprintf(stderr, "Error connecting to %s: %s\n",
home, wiredtiger_strerror(ret));

The effectiveness of the chosen cache size can be measured by reviewing the page eviction statistics for the database.

Cache resident objects

Objects can be created as cache resident - that is their contents will remain in cache and never be considered for the purposes of cache eviction. Cache residence can be configured with the WT_SESSION::create "cache_resident" configuration string. LSM tree objects do not support the "cache_resident" setting.

Configuring a cache resident object has several effects:

  • Once the object's pages have been created or instantiated in memory no further I/O cost is ever paid for object access, minimizing potential latency.
  • Cache resident objects can be accessed faster than objects tracked for potential eviction.
  • If cache resident objects require a significant proportion of the configured cache size then non cache-resident objects can incur significantly higher I/O churn.
  • If cache resident objects require more space than the configured cache size, then further operations will either return error or stall until space is made available by closing objects.

An example of configuring a cache-resident object:

ret = session->create(session,
"table:mytable", "key_format=r,value_format=S,cache_resident=true");

Eviction tuning

When an application approaches the maximum cache size, WiredTiger begins eviction to stop memory use from growing too large, approximating a least-recently-used algorithm.

WiredTiger provides several configuration options for tuning how pages are evicted from the cache. Different settings will improve performance depending on an application's particular workload. Customizing the eviction configuration settings can reduce latency spikes in application threads and can improve throughput in some applications.

WiredTiger eviction tuning options can be configured when first opening a database via wiredtiger_open, or changed after open with WT_CONNECTION::reconfigure.

The eviction_trigger configuration value is the occupied percentage of the total cache size that causes eviction to start. By default, WiredTiger begins evicting pages when the cache is 95% full. An application concerned about a latency spike as the cache becomes full might want to begin eviction earlier.

The eviction_target configuration value is the overall target for eviction, expressed as a percentage of total cache size; that is, once eviction begins, it will proceed until the target percentage of bytes in the cache is reached. Note the eviction_target configuration value is ignored until eviction is triggered.

The eviction_dirty_target configuration value is the overall dirty byte target for eviction, expressed as a percentage of total cache size; that is, once eviction begins, it will proceed until the target percentage of dirty bytes in the cache is reached. Note the eviction_dirty_target configuration value is ignored until eviction is triggered.

* Configure eviction to begin at 90% full, and run until the cache
* is only 75% dirty.
ret = wiredtiger_open(home, NULL,
"create,eviction_trigger=90,eviction_dirty_target=75", &conn);

By default, WiredTiger cache eviction is handled by a single, separate thread. In a large, busy cache, a single thread will be insufficient (especially when the eviction thread must wait for I/O). The eviction=(threads_min) and eviction=(threads_max) configuration values can be used to configure the minimum and maximum number of additional threads WiredTiger will create to keep up with the application eviction load. Finally, if the Wiredtiger eviction threads are unable to keep up with application demand for cache space, application threads will be tasked with eviction as well, potentially resulting in latency spikes.

/* Configure up to four eviction threads */
ret = wiredtiger_open(home, NULL,
"create,eviction_trigger=90,eviction=(threads_max=4)", &conn);