[ofiwg] allowing aliasing in fetch atomics?

[Jeff Hammond]

It is cheap for contiguous buffers.  It may not be cheap for iovecs.  I
don't know what the restrictions on iovecs are, but I wonder if the alias
check is O(count^2) for the iov functions.  Jim Dinan wrote an optimized
version of alias checking in ARMCI-MPI; it may be better than O(n^2).  In
any case, it is added complexity for a questionable use case.

Jeff

On Wed, Nov 11, 2015 at 12:12 PM, Sur, Sayantan <sayantan.sur at intel.com>
wrote:

> In either case, we should clarify in the man pages (if not already clear)
> about aliasing. I agree with Jeff that if there is no need for atomics
> using aliased buffers, then we shouldn’t define it either. I know the check
> for aliased buffers is cheap, but why have that check at all, if no use
> case exists?
>
> From: <ofiwg-bounces at lists.openfabrics.org> on behalf of Jeff Hammond <
> jeff.science at gmail.com>
> Date: Wednesday, November 11, 2015 at 12:07 PM
> To: "Dave Goodell (dgoodell)" <dgoodell at cisco.com>
> Cc: "ofiwg at lists.openfabrics.org" <ofiwg at lists.openfabrics.org>
> Subject: Re: [ofiwg] allowing aliasing in fetch atomics?
>
> Sure, and every C compiler out there can do multiple version code
> generation such that no code should ever show a benefit with restrict.
>
> The fundamental idea here is that allowing aliasing is a bad semantic and
> there should be a compelling reason for supporting it.  The higher level
> models that informed the design of OFI either explicitly prohibit aliasing
> (MPI RMA and UPC) or their APIs make it impossible (SHMEM).
>
> If there is client that requires this semantic, I'd like to understand why
> it is permitted to burden OFI rather than requiring that client to do its
> own buffering.
>
> I am still investing Jianxin's comment on the ticket wherein MPI was
> affected by this.  I do not understand how a correct MPI program would
> encounter an issue here.
>
> Jeff
>
> On Wed, Nov 11, 2015 at 11:34 AM, Dave Goodell (dgoodell) <
> dgoodell at cisco.com> wrote:
>
>> On Nov 11, 2015, at 2:01 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
>> >
>> > Why do memcpy and memmove both exist?  Why did C99 introduce restrict?
>> Why does Fortran prohibit aliasing?
>> >
>> > I have measured the benefits of restrict semantics w.r.t. vectorization
>> many times in the past, enough that I did not bother to benchmark this
>> specific case.
>>
>> But you already offered a fix for that issue, which was to just have two
>> versions of the implementation to handle aliased/non-aliased.  The overlap
>> check is pretty cheap.
>>
>> -Dave
>>
>>
>
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
>
>


-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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