Everyone’s got their pet peeves: For Poempelfox it’s Schiphol Airport, for Andreas Stiller it’s the infamous A20 gate. Compared to those glorious fails, my favorite tech blunder is a rather measly one, and it may not be relevant to many users in practice. However, it’s not so much the importance of it but the total mystery of how it came to happen. So here’s the thing.
Loads, stores, and AGUs
Sandy Bridge and Ivy Bridge LOAD and STORE units, AGUs, and their respective ports.
The Intel Sandy Bridge and Ivy Bridge architectures have six execution ports, two of which (#2 & #3) feed one LOAD pipeline each and one (#4) feeds a STORE pipe. These units are capable of transferring 16 bytes of data per cycle each. With AVX code, the core is thus able to sustain one full-width 32-byte LOAD (in two adjacent 16-byte chunks) and one half of a 32-byte STORE per cycle. But the LOAD and STORE ports are not the only thing that’s needed to execute these instructions – the core must also generate the corresponding memory addresses, which can be rather complicated. In a LOAD instruction like:
vmovupd ymm0, [rdx+rsi*8+32]
the memory address calculation involves two integer add operations and a shift. It is the task of the address generation units (AGUs) to do this. Each of ports 2 and 3 serves an AGU in addition to the LOAD unit, so the core can generate two addresses per cycle – more than enough to sustain the maximum LOAD and STORE throughput with AVX.
The peculiar configuration of LOAD and STORE units and AGUs causes some strange effects. For instance, if we execute the Schönauer vector triad: Continue reading →
MachineState is a python3 module and CLI application for documenting and comparing settings known to affect application performance: e.g., CPU/Uncore frequencies, hardware prefetchers, memory capacity, but also OS and software settings like NUMA balancing, writeback workqueues, scheduling, or the versions of common tools and libraries (e.g., compilers and MPI). All this information can be essential for reproduction of benchmark results. The MachineState tool gathers all (known) settings and presents them as a JSON document. A state file written earlier can be compared to the current machine state to uncover deviations from the original test system.
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