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General CPU Discussion / Re: Performance of HPT vs Radix?
« on: February 21, 2025, 06:37:52 pm »
This may be helpful: https://www.researchgate.net/publication/325937212_IBM_POWER9_system_software
"The HPT has the advantage that a translation can be performed (i.e., a translation lookaside buffer [TLB] miss can be serviced) by reading up to two cache lines from memory. This characteristic should enable the HPT to provide good performance for applications with very large memory footprints and low locality of references (i.e., with essentially random or quasi-random access patterns).
"The disadvantage of the HPT structure is that it does not cache well. The hashing algorithm used in the POWER Memory Management Unit (MMU) tends to put each page table entry (PTE) for a process into a separate cache line. Thus, most TLB misses cause a cache miss and need to read main memory, particularly in processes with large working sets. ...
"Because radix page tables place information about adjacent addresses into adjacent doublewords in memory, a program with high locality of references in its address access pattern will also have high locality of references in the pattern of accesses performed by the MMU to service TLB misses. Thus, the CPU caches work efficiently to cache the page-table entries (PTEs), and so radix tree translation is expected to be more efficient than HPT translation for workloads with high locality of references."
IME, for many workloads, the difference between HPT and radix MMU is imperceptible, and it does have some impact on what you can virtualize. On the other hand, for large working sets typical of IBM midrange and "big iron," radix does have real benefits.
"The HPT has the advantage that a translation can be performed (i.e., a translation lookaside buffer [TLB] miss can be serviced) by reading up to two cache lines from memory. This characteristic should enable the HPT to provide good performance for applications with very large memory footprints and low locality of references (i.e., with essentially random or quasi-random access patterns).
"The disadvantage of the HPT structure is that it does not cache well. The hashing algorithm used in the POWER Memory Management Unit (MMU) tends to put each page table entry (PTE) for a process into a separate cache line. Thus, most TLB misses cause a cache miss and need to read main memory, particularly in processes with large working sets. ...
"Because radix page tables place information about adjacent addresses into adjacent doublewords in memory, a program with high locality of references in its address access pattern will also have high locality of references in the pattern of accesses performed by the MMU to service TLB misses. Thus, the CPU caches work efficiently to cache the page-table entries (PTEs), and so radix tree translation is expected to be more efficient than HPT translation for workloads with high locality of references."
IME, for many workloads, the difference between HPT and radix MMU is imperceptible, and it does have some impact on what you can virtualize. On the other hand, for large working sets typical of IBM midrange and "big iron," radix does have real benefits.
