[openib-general] [PATCH 1/2] mthca support for max_map_per_fmr device attribute

[Or Gerlitz]

Or Gerlitz wrote:
> The max fmr remaps device attribute is not set by the driver, so the generic 
> fmr_pool uses a default of 32. Enlaring this quantity would make the amortized 
> cost of remaps lower. With the current mthca "default profile" on memfull HCA 
> 17 bits are used for MPT addressing so an FMR can be remapped 2^15 - 1 >> 32 times.

Actually, the bigger (than unmap amortized cost) problem i was facing 
with the unmap count being very low is the following: say my app 
publishes N credits and serving each credit consumes one FMR, so my app 
implementation created the pool with 2N FMRs and set the watermark to N.

When "requests" come fast enough, there's a window in time when there's 
an unmapping of N FMRs running at batch, but out of the remaining N FMRs 
some are already dirty and can't be used to serve a credit. So the app 
fails temporally... So, setting the watermark to 0.5N might solve this, 
but since enlarging the number of remaps is trivial, i'd like to do it 
first.

The app i am talking about is a SCSI LLD (eg iSER, SRP) where each SCSI 
command consumes one FMR and the LLD posts to the SCSI ML how many 
commands can be issued in parallel.

Or.