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kriesel 2020-12-30 00:16

Xeon Phi
This is intended as a reference thread. Do not post here. Post comments at [URL][/URL] instead. Posts placed here may be moved or deleted without warning or recourse. Note also, some small bits of this may have already appeared in other threads on the forum, but are reproduced here without linking for convenience of the reader.

This is a draft in progress

Xeon Phi is a manycore Intel x86 design (57 to 72 cores per socket). [URL][/URL]
Some of the approaches demonstrated first in the Xeon Phi product line have been incorporated into more mainstream processors.

There were five generations announced:
Knights Ferry
Knights Corner
Knights Landing 2nd generation MIC
Knights Hill (canceled, no indication it shipped; intended as 3rd generation MIC)
Knights Mill [URL][/URL]

Knights Corner and Knights Landing are currently (2020) appearing regularly on the used resale market, so are of possible interest to hobbyists. Knights Corner has an interestingly different instruction set. It's likely to have limited compatibility with GIMPS applications. I haven't been able to test that yet.
Knights Landing is the beginning of AVX512F support. Some past development has been performed on Knights Landing for mprime/prime95 and Mlucas at least. So compatibility should be good there, and perhaps Mfactor as well.

[B]Selecting hardware[/B]

An old forum thread discusses the Knight's Corner. [URL][/URL]

There was a crowd-sourced purchase of a Knight's Landing system for GIMPS a few years ago. [URL][/URL] This had the advantage of coming preconfigured with OS and tools. A few photos are at [URL][/URL] Some early mprime benchmark data are at [URL=""],[/URL] [URL][/URL], [URL][/URL] before optimizing for numerous variables including those listed in [URL][/URL]
An article from Colfax Research discusses Knights' Landing memory models. [URL][/URL]

Knight's Corner generation are much more economical than Knight's Landing which has nearly triple the DP performance. Knight's Corner is not full AVX512 implementation, while Knights Landing includes AVX512F. For other differences, see [URL][/URL]
Knight's Corner are mostly PCIe based. Most models end in P indicating they are designed to be cooled passively by separate case-mounted fans in servers or supercomputer designs. An example is the 31S1P, photos [URL=""]here[/URL]. But there are 3120A and 7120A models that are PCIe with integral fans; 7120A photos [URL=""]here[/URL]. Used 3120A or 7120A units are available sometimes below $200 in 2020. These can be estimated by stated DP performance and memory bandwidth to be perhaps 1/5 the performance of a Radeon VII in primality testing or similar computation.
Pricing and form factor were decisive for me, for a first foray into Xeon Phi, so I bought a 7120A (Knights Corner) when I saw one available at a good price, $125+tax in spring 2020. It's nominally 225W so likely poor performance/power-cost. Other priorities intervened and delayed its installation

Knight's Landing are mostly socket-style, for use as the cpu in a system with a special motherboard designed for them, but an occasional PCIe-format coprocessor unit surfaces. Prices of used units tend to range from $600 to above $2000 for Knight's Landing, as of mid 2020. These are about 1/2 the performance of a Radeon VII in primality testing or similar computation. Recently bare-bones systems have been offered for $500 to $700 plus tax and shipping. Occasionally a 7220P PCIe coprocessor will be offered for sale used. In the last few days of 2020 there's an eBay seller in Australia offering what appears to be 3d-printed adapters with blowers.

The price/performance of either Knights Corner or Knights Landing is not compelling compared to a new or recent gpu model for mainstream GIMPS work. But the memory capacity is, if it's usable, particularly for P-1 stage 2 on large exponents (such as F33, or gigabit or larger Mersennes). Systems based on Knights Landing with several DIMM slots to augment the on-package MCDRAM can contain up to a total of 400GB of ram, far beyond the 16GB or less typical of affordable gpus in 2020. The Knights Landing based systems are particularly appealing because they are general purpose computers that happen to have a LOT of cores, and are straightforward to get up and running, compared to the complexities of dealing with a coprocessor. The space constraints of dual-slot-PCIe format limit cooling so limit performance and features.

Knights Corner reference thread [URL][/URL]
Knights Landing reference thread tbd

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