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Porting memutil

Evaluating the ideal frequency.

First it is best to run the Evaluations to get a sense of which workloads perform best at which frequencies on your machine.

Modifying and Running Memutil

Memutil with the l2stalls heuristic is currently tuned for a laptop based on the i7-6700HQ mobile processor.

To improve performance of memutil on your machine, first make sure you can compile memutil for your kernel and architecture (Intel and AMD only currently).

Then to choose the heuristic.

Using L2-cache stalls

This is the default heuristic used and performs best. However the needed performance counters are not always available. As default the counter cycle_activity.stalls_l2_miss is used. If this counter is not available perf list can be used to get a list of available counters and search for a replacement. The alternative counter name can be passed to memutil as parameter as in insmod memutil.ko event_name3=<replacement counter name>. Note that it has to be event_name3 as event_name1 and event_name2 are used for other counters.

Using IPC

This heuristic peforms a bit worse, however it is usable on each computer as it does not use any platform specific counters. To use this heuristic the line #define HEURISTIC HEURISTIC_OFFCORE_STALLS in memutil_main.c has to be changed to #define HEURISTIC HEURISTIC_IPC.

Tuning the heuristic

This section assumes that you read the heuristics page, especially the implementation details (here)

Both heuristics use a linear interpolation for which the parameters $\beta_{max}$ and $\beta_{min}$ can, and probably should, be tuned by passing their value as module parameter.

HEURISTIC_IPC

Here $\beta_{max}$ is called max_ipc and if the IPC reaches this value the maximum frequency is chosen. $\beta_{min}$ is called min_ipc. If that IPC is reached, the minimum frequency will be chosen. Note that these values are given in percent (i.e. $\beta_{min} = \frac{\text{min\_ipc}}{100}$) and should be between 0 and 100. Also min_ipc should be smaller than max_ipc.

To pass these values to memutil, insert the module as follows: insmod memutil.ko max_ipc=<some_value> min_ipc=<some_value>.

It is best to repeatedly insert memutil with different values, then run both an on-core and an off-core bound workload and check whether memutil chooses an appropriate frequency.

If memutil chooses a wrong frequency for the off-core (i.e. memory-bound) workload, adjust the min_ipc. Increasing the value should reduce the frequency, lowering it should increase the frequency. The same goes for on-core (i.e. CPU-bound) workloads and the max_ipc value.

HEURISTIC_OFFCORE_STALLS

Here $\beta_{max}$ is called max_stalls_per_cycle and if the L2-stalls/cycle reaches this value the minimum frequency is chosen. $\beta_{min}$ is called min_stalls_per_cycle. If that value is reached, the maximum frequency will be chosen. Note that these values are given in percent (i.e. $\beta_{min} = \frac{\text{min\_stalls\_per\_cycle}}{100}$) and should be between 0 and 100. Also min_stalls_per_cycle should be smaller than max_stalls_per_cycle.

To pass these values to memutil, insert the module as follows: insmod memutil.ko max_stalls_per_cycle=<some_value> min_stalls_per_cycle=<some_value>.

It is best to repeatedly insert memutil with different values, then run both an on-core and an off-core bound workload and check whether memutil chooses an appropriate frequency.

If memutil chooses a wrong frequency for the off-core (i.e. memory-bound) workload, adjust the max_stalls_per_cycle. Decreasing the value should reduce the frequency, increasing it should increase the frequency. The same goes for on-core (i.e. CPU-bound) workloads and the min_stalls_per_cycle value: Increasing it will increase the frequency while decreasing it will reduce the frequency.