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> 9x bandwidth to RAM (230 GB/s vs. 25.6) Haswell-EP supports 4 DDR4-2133 channels, or 68 GB/s (theoretical). TDP on POWER8 is almost double and it's much mor
by jedbrown 12y ago
> 9x bandwidth to RAM (230 GB/s vs. 25.6)
Haswell-EP supports 4 DDR4-2133 channels, or 68 GB/s (theoretical). TDP on POWER8 is almost double and it's much more expensive, so we should think about it as having perhaps double the DRAM bandwidth (still excellent).
Note that POWER8 gets this bandwidth through many more channels, which favors the use of many memory streams. This has been the case historically, with POWER favoring data structures that result in many streams, while the same transformation has been catastrophic for memory performance on Blue Gene's PPC, with its limited prefetch engine and small number of outstanding memory requests.
- rdtsc 12y agoThat is interesting. RAM bandwidth could significantly impact a lot of applications. I guess the only ways it to measure and compare. The problem I see with power is it has a different endiannes and even though compilers know how to handle it, a lot of libraries and code might just assume some specific ordering (little endian) and thus fail un-expectedly on POWER.
- wmf 12y agoThat's why Power is now little-endian.
- gsnedders 12y agoAs far as I'm aware, IBM intend on maintaining both big and little endian Linux ports (and toolchains).
- PhuFighter 12y agoPower has been little endian for a while. No one bothered to tell the software guys to write a little endian OS and tools for it....
- wmf 12y agoMy point is that little-endian Ubuntu is now available.
- justincormack 12y agoYes, and IBM is pushing this (plus the v2 ABI which is a bit more like x86 and simplified in some small ways).
- rdtsc 12y agoI had no idea. Haven't looked it since many years ago. Well, heck, that's great then.
- pavlov 12y agoWasn't PowerPC always able to switch between little-endian and big-endian at boot? Maybe they migrated that capability to POWER. (It's been a long time since I last typed "PowerPC", gave me a strange feeling of nostalgia...)
- apaprocki 12y agoPOWER architecture can switch between BE/LE via a special purpose register (SPR). This can even be done to support mixing BE/LE threads on the core. The software complexity of managing that, though, has made them decide to enable the entire OS as BE or LE. One cool thing is that the support will be extended so that the chips can support running either BE or LE guest VMs natively. edit: Straight from IBM https://www.ibm.com/developerworks/community/blogs/fe313521-2e95-46f2-817d-44a4f27eba32/entry/just_the_faqs_about_little_endian?lang=en https://www.ibm.com/developerworks/community/blogs/fe313521-...
- justincormack 12y agoMost of them, although not the Apple G5s which are BE only.
- davidw 12y agoA lot of those problems were solved years ago. I remember a big push in the late 90ies, on Debian, to ensure proper endianness, because of the many architectures on which it ran.