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Subject: Re: [OMPI devel] [ofa-general] uDAPL EVD queue length issue
From: Jon Mason (jon_at_[hidden])
Date: 2007-12-04 19:57:15


On Tue, Dec 04, 2007 at 11:40:17AM -0800, Arlin Davis wrote:
> Jon Mason wrote:
>> While working on OMPI udapl btl, I have noticed some "interesting"
>> behavior. OFA udapl wants the evd queues to be a power of 2 and
>> then will subtract 1 for book keeping (ie, so that internal head and
>> tail pointers never touch except when the ring is empty). OFA udapl
>> will report the queue length as this number (and not the original
>> size requested) when queried. This becomes interesting when a power
>> of 2 is passed in and then queried. For example, a requested queue
>> of length 256 will report a length of 255 when queried.
>
> Something is not right. You should ALWAYS get at least what you request. On
> my system with an mthca, a request of 256 gets you 511. It is the verbs
> provider that is rounding up, not uDAPL.
>
> Here is my uDAPL debug output (DAPL_DBG_TYPE=0xffff) using dtest:
>
> cq_object_create: (0x519bb0,0x519d00)
> dapls_ib_cq_alloc: evd 0x519bb0 cqlen=256
> dapls_ib_cq_alloc: new_cq 0x519d60 cqlen=511
>
> This is before and after the ibv_create_cq call. uDAPL builds it's EVD
> resources based on what is returned from this call.
>
> I modified dtest to double check the dat_evd_query and I get the same:
>
> 8962 dto_rcv_evd created 0x519e80
> 8962 dto_req_evd QLEN - requested 256 and actual 511
>
> What OFED release and device are you using?

I'm running OFED 1.2.5 and using Chelsio.

The behavior of the iwch_create_cq in
drivers/infiniband/hw/cxgb3/iwch_provider.c is to allocate the amount
given (rounded to the power of 2). So this function will give 256 if
256 is requested, but uDAPL will consume one of those for book keeping
and thus only have 255.

For my clarification, the provider should take into account the
bookkeeping of uDAPL and roundup to the next power of 2 when given a
power of 2 size? I'm probably being thick, but why doesn't uDAPL
increase the size requested by one before passing the request to the
provider (or is this the documented behavior of the function and the
provider should conform)?

Thanks,
Jon

>
> -arlin
>
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>
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