Re: [PATCH v5 0/5] DMA mapping changes for SCSI core

From: John Garry
Date: Mon Jul 11 2022 - 03:37:04 EST

On 11/07/2022 00:08, Damien Le Moal wrote:
Ah, I think that I misunderstood Damien's question. I thought he was
asking why not keep shost max_sectors at dma_max_mapping_size() and then
init each sdev request queue max hw sectors at dma_opt_mapping_size().
I was suggesting the reverse:) Keep the device hard limit
(max_hw_sectors) to the max dma mapping and set the soft limit
(max_sectors) to the optimal dma mapping size.

Sure, but as I mentioned below, I only see a small % of requests whose mapping size exceeds max_sectors but that still causes a big performance hit. So that is why I want to set the hard limit as the optimal dma mapping size.

Indeed, the IOMMU IOVA caching limit is already the same as default max_sectors for the disks in my system - 128Kb for 4k page size.

But he seems that you want to know why not have the request queue max
sectors at dma_opt_mapping_size(). The answer is related to meaning of
dma_opt_mapping_size(). If we get any mappings which exceed this size
then it can have a big dma mapping performance hit. So I set max hw
sectors at this ‘opt’ mapping size to ensure that we get no mappings
which exceed this size. Indeed, I think max sectors is 128Kb today for
my host, which would be same as dma_opt_mapping_size() value with an
IOMMU enabled. And I find that only a small % of request size may exceed
this 128kb size, but it still has a big performance impact.