64-bit DMA performance benefits
On Power® Systems servers, certain PCIe slots support 64-bit direct memory access (DMA), which improves I/O operations so that there is less system usage and better performance.
- perf JSON l2_* metric events failing in POWER9™
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In POWER8® and earlier generation processors, both bus events and direct events of performance monitoring unit can be individually programmed and monitored in PMCs. However, in POWER9, L2 and L3 bus events are available as a group of four events. To obtain the counts for any of the L2 or L3 bus event in a particular group, you must program PMC4 with corresponding L2 or L3 bus event for that group. Since perf JSON metric events run as an individual event, l2_* JSON metric events present in POWER9 event list fails because these JSON metric events run as L2 and L3 bus events.
Following are the constraints for L2 or L3 bus events in POWER9:- When any L2 or L3 event is programmed, it is expected to program corresponding PMC4 event from that group.
- PMC4 event must be programmed first due to group constraint logic limitation.
The JSON metric events in perf run as individual events and not as a group, hence failing of perf JSON 12_* metric event is expected in POWER9 systems. Currently, no workaround is available for the perf JSON l2_* metric events failing in POWER9.