The v_repage_hi parameter

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The v_repage_hi parameter controls the threshold defining memory overcommitment. Memory load control attempts to suspend processes when this threshold is exceeded during any one-second period.

The threshold is a relationship between two direct measures: the number of pages written to paging space in the last second (po) and the number of page steals occurring in the last second (fr). You can see both these values in the vmstat output. The number of page writes is usually much less than the number of page steals. Memory is overcommitted when the following is true:

po/fr > 1/v_repage_hi or po*v_repage_hi > fr

The schedo -o v_repage_hi=0 command effectively disables memory load control. If a system has at least 128 MB of memory, the default value is 0, otherwise the default value is 6. With at least 128 MB of RAM, the normal VMM algorithms usually correct thrashing conditions on the average more efficiently than by using memory load control.

In some specialized situations, it might be appropriate to disable memory load control from the outset. For example, if you are using a terminal emulator with a time-out feature to simulate a multiuser workload, memory load control intervention may result in some responses being delayed long enough for the process to be killed by the time-out feature. Another example is, if you are using the rmss command to investigate the effects of reduced memory sizes, disable memory load control to avoid interference with your measurement.

If disabling memory load control results in more, rather than fewer, thrashing situations (with correspondingly poorer responsiveness), then memory load control is playing an active and supportive role in your system. Tuning the memory load control parameters then may result in improved performance or you may need to add RAM.

A lower value of v_repage_hi raises the thrashing detection threshold; that is, the system is allowed to come closer to thrashing before processes are suspended. Regardless of the system configuration, when the above po/fr fraction is low, thrashing is unlikely.

To alter the threshold to 4, enter the following:

# schedo -o v_repage_hi=4

In this way, you permit the system to come closer to thrashing before the algorithm starts suspending processes.