Memory use in the MobiLink server

The MobiLink server primarily uses memory in the following ways:

caching
for VMs
storing system data

Cache

In a typical MobiLink server, most of the memory is used by the memory cache, which stores row data and related data structures and network buffers. In general, data that is unbounded in either size or quantity is stored in the cache. If the amount of data to store in the memory cache exceeds the cache's size the excess is transferred to disk, which can become a potential bottleneck in synchronization performance. The degree to which swapping to disk is a problem depends on the amount and frequency of disk I/O and the speed of the disk. To eliminate this potential bottleneck, it is recommended that you avoid swapping data to disk completely. Watch for warning 10082 in the MobiLink server log, or the "Cache is full" alert in the SQL Anywhere Monitor.

By default, the MobiLink server automatically grows its cache up to 60% of the available process address space and shrinks its cache if other processes on the system require more memory, or the server's non-cache memory grows larger. The initial maximum and minimum cache sizes can be controlled with the -cinit, -cmax and -cmin mlsrv16 server options. If desired, the server can disable dynamic cache sizing by specifying the same value for the -cmax and -cmin option values.

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There are several ways to find out information about the cache:

Server Cache Size This is a SQL Anywhere Monitor metric that gives the current cache size. See List of metrics for MobiLink server resources.
Percent of Pages Locked This is a SQL Anywhere Monitor metric that gives the percentage of pages in the cache that cannot be transferred to disk. See List of metrics for MobiLink server resources.
Percent of Pages Used This is a SQL Anywhere Monitor metric that gives the percentage of pages that contain valid data. See List of metrics for MobiLink server resources.
PAGES_LOCKED_MAX The -ppv option for mlsrv16 can print the number of pages in the memory cache. See -ppv mlsrv16 option.
PAGES_LOCKED The -ppv option for mlsrv16 can print the number of cache pages loaded into memory. See -ppv mlsrv16 option.
PAGES_USED The -ppv option for mlsrv16 can print the number of cache pages used. See -ppv mlsrv16 option.
-vk option for mlsrv16 Causes the server to print a line to the log when the cache grows or shrinks. See -ppv mlsrv16 option.

VMs

Another major use of memory within the MobiLink server is the embedded Java and .NET VMs. In deployments that make heavy use of the direct row API or that embed application servers like JBoss or Tomcat, the VM memory use can exceed the memory cache.

When using a Java VM, you usually have some control over the amount of memory the VM uses. Most Java VMs provide the -Xms and -Xmx options that allow you to specify the maximum and initial VM heap size, respectively.

You can find out the amount of memory being used by the embedded VMs with the VM Memory Usage SQL Anywhere Monitor metric, or with the VM_MEM_USE -ppv value.

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Other memory usage

The remaining memory is used for storing state information about each synchronization, thread stacks, inter-thread communication and the ODBC driver. Typically, this accounts for a few tens of megabytes of memory and the amount used is bounded by the -nc, -sm and -w options for mlsrv16. It is worth noting that non-SQL Anywhere ODBC drivers can use a significant amount of memory, particularly when processing BLOB columns.

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