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Subject: [OMPI devel] Remote key sizes
From: Rolf vandeVaart (rvandevaart_at_[hidden])
Date: 2011-11-08 09:36:03


> george.
>
>PS: Regarding the hand-copy instead of the memcpy, we tried to avoid using
>memcpy in performance critical codes, especially when we know the size of
>the data and the alignment. This relieves the compiler of adding ugly intrinsics,
>allowing it to nicely pipeline to load/stores. Anyway, with both approaches
>you will copy more data than needed for all BTLs except uGNI.

I was looking at a case in a BTL I was working on where I actually need 64 bytes (yes, bytes) as the remote key size as opposed to the current 16 bytes (128 bits).
Not sure how I can handle that yet. (I assume configure is my friend, but even in that case, all headers will need to carry around the extra data.)

Rolf

>
>On Nov 7, 2011, at 21:48 , Nathan T. Hjelm wrote:
>
>>
>>
>> On Mon, 7 Nov 2011 17:18:42 -0500, George Bosilca
>> <bosilca_at_[hidden]>
>> wrote:
>>> A little bit of history:
>>>
>>> 1. r25305: added 2 atomic operations to OPAL. However, they only
>>> exists
>> on
>>> amd64 and are only used in the vader BTL, which I assume only
>>> supports amd64.
>>
>> Two things:
>> - The atomic is a new feature that has no impact on existing code. It
>> can also be implemented on Intel but we have not tested it (yet).
>> - The atomic was pushed to support lock-free queues in the Vader BTL.
>> Vader does not need the atomics and can use an atomic lock lock but I
>> see higher latencies when using locks.
>>
>> Why would this change (that has no impact on any other code) need an
>RFC?
>>
>>> 2. r25334: The seg_key union got a new member ptr. This member is
>>> solely used in the vader BTL, as all other BTL use a compiler trick
>>> to convert a pointer to a 64 bits.
>>
>> I am actually going to remove that member. I prefer the use of
>> uintptr_t over casting to a uint64_t but it has no real benefit and
>> possibly a pitfall due to its platform dependent size.
>>
>> But the member has, like the atomic, no impact on any exiting code. It
>> does not change the size of the seg_key and was only used by Vader.
>> Why would this change have required an RFC?
>>
>>> 3. r25445: All members of the seg_key union got friends, because Cray
>> dare
>>> to set their keys at 128 bits long. However a quick find . -name
>>> "*.[ch]" -exec grep -Hn seg_key {} \; | grep "\[1\]"
>>> indicates that no BTL is using 128 bits keys. Code has been added to
>>> all PMLs, but I guess they just copy empty data.
>>
>> For now they copy empty data but in the near future (as I have said)
>> we will need to bits for the ugni btl (Cray XE Gemini). I pushed this
>> code to prepare for pushing ugni.
>>
>> Also, you might be a good person to ask: Why do we copy each member of
>> a segment individually in the PMLs? Wouldn't it be faster to do a
>> memcpy? If we were using a memcpy I would not have had to make any
>change to the pmls.
>>
>>> What I see is a pattern of commits that can have been dealt with
>>> differently. None had an RFC, and most of them are not even used.
>>
>> I think you are reaching a little here. I pushed several changes over
>> a period of a month. The first two are not related to the third which
>> is the only one that could have any impact to existing code and might
>> require an RFC.
>>
>> In retrospect I should have done a RFC for the 3rd change with a short
>> timeout. At the time (operating on little sleep) it seemed like the
>> commits would have minimal impact. Please let me know if the commits
>> have any negative impact.
>>
>> -Nathan
>>
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>
>
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