Trying to find a PCIe mobo

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But with at least 3 4-lane PCIe sockets (that can all operate at 4-lanes at
the same time) and has a TLP payload size of 4096 bytes.

I've got a Gigabyte EP45-DS3P which is a good board. It has the required
number of slots, but the payload is only 128 bytes max.

Anyone got any ideas?

Re: Trying to find a PCIe mobo

Grumps wrote:
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I recollect seeing mention of PCI Express payload size in some
earlier BIOS screens. But doing a search right now, is indicating
what the hardware is doing, is something quite different.

This article, would have you believe that you just turn a knob,
and by magic, that is what you get. I got a similar impression
by reading BIOS manuals, that the hardware really could support
all those values.

Now, if I look at some actual hardware company documentation,
the numbers quoted are quite different.

    "Currently most if not all devices available on the market
     support 256 byte MPS values or less"

    "1.1 Market Segmentation of Maximum Payload Support

     We observe distinct market segmentation in the support for
     various maximum payload values. Intel desktop chipsets support
     at most a 64-byte maximum payload while Intel server chipsets
     support at most a 128-byte maximum payload. The primary reason
     for this is to match the cache line size for snooping on the front
     side bus. A secondary reason may be that the memory controller
     itself is optimized around handling cache line sizes. Finally,
     the buffer memory required is roughly proportional to the maximum
     payload size; supporting longer packets raises device cost. The
     majority of the market is well served with a maximum payload of
     256 bytes or less.

     Chipsets produced by vendors other than Intel have supported a
     higher value; 512 bytes is the commonly known maximum payload
     value for a server North Bridge. As will be shown later, this
     value provides higher throughput for storage and network traffic
     and in fact seems to mark the point of diminishing returns, except
     for a specialized storage infrastructure.

     The storage infrastructure is optimized to transfer long files
     segmented into disk sector size payloads of 2K- or 4K-bytes.
     Storage ASSPs typically support payloads of up to 4 K-bytes and
     in most cases exhibit less than optimal performance with shorter
     packet lengths due to internal architectural tradeoffs and lack
     of large packet-size support in other components in the path of
     transfer. This is the case for storage “boxes” as opposed to
     storage HBAs that are limited by the North Bridge’s maximum payload
     capability. Most storage OEMs have adopted to short payload
     restrictions but a small percentage of systems using proprietary
     North Bridge function through custom ASICs have been consistent
     in requiring support for longer packet lengths."

The reason I decided to look that stuff up, is I was having
trouble finding a board to match your requirement. It isn't a
problem finding a bandwidth monster, in terms of lanes, but
the BIOS setting in question has largely disappeared from the
latest generation of BIOS. (PEG buffer size)

I'd probably try something with a 790i, as a way to get
some bandwidth. This one has wiring for (3) x16 and (1) x8,
which meets your (4) x4 requirement. But I don't know what
the answer is to your TLP requirement.

P5N64 WS Professional


Re: Trying to find a PCIe mobo

Paul wrote:
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Quoted text here. Click to load it

Thanks Paul.

I read those xilinx and plxtech pages before and was quite surprised. A 64
byte payload!

To be honest, our current 128 byte max payload isn't causing us problems
yet, and the extra efficiency of going to the max PCIe spec of 4kB probably
won't make much difference for us. But I just thought that if a mobo where
the max was available, then that would be the better option (all other thing
being equal, which they never are!).

That Asus board (and I seem to remember having to sit down when I saw the
price a short while ago), was briefly on our shortlist. After reading the
manual, I'll give them a shout and see what the payload is.

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