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Subject: Re: [OMPI devel] knem_dma_min
From: Brice Goglin (Brice.Goglin_at_[hidden])
Date: 2010-08-18 16:54:25


Le 18/08/2010 19:21, Eugene Loh a écrit :
> Eugene Loh wrote:
>
>> In mca_btl_sm_get_sync(), I see this:
>> /* Use the DMA flag if knem supports it *and* the segment length
>> is greater than the cutoff. Note that if the knem_dma_min
>> value is 0 (i.e., the MCA param was set to 0), the segment size
>> will never be larger than it, so DMA will never be used. */
>> icopy.flags = 0;
>> if (mca_btl_sm_component.knem_dma_min <= dst->seg_len) {
>> icopy.flags = mca_btl_sm_component.knem_dma_flag;
>> }
>>
>> I'm going to poke around some more, but this doesn't on the surface
>> make sense to me. If knem_dma_min==0, it would seem as though the
>> segment size will *always* be at least that large and DMA will
>> *always* be used (if supported).
>
> Answering my own question (or guessing, in any case), maybe the code
> is okay but the comment is misleading. If knem_dma_min==0, then
> mca_btl_sm_component_init() sets knem_dma_flag to 0. So, the seg_len
> test passes, but it has no effect.

Yes, your understanding is correct. It looks like the comment was not
properly updated last time we changed the meaning of knem_dma_min.

Brice