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Subject: Re: [OMPI devel] knem_dma_min
From: Eugene Loh (eugene.loh_at_[hidden])
Date: 2010-08-18 13:21:47

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.