Re: [PATCH v1 2/2] dma-mapping-common: add DMA attribute - DMA_ATTR_IOMMU_BYPASS

Shamir Rabinovitch
2015-11-07T15:06:42Z

On Thu, Nov 05, 2015 at 04:11:21PM -0500, David Miller wrote:
>
> And for the record Sowmini fixed a lot of the lock contention:
>
> commit ff7d37a502022149655c18035b99a53391be0383
> Author: Sowmini Varadhan <sowmini.varadhan@oracle.com>
> Date: Thu Apr 9 15:33:30 2015 -0400
>
> Break up monolithic iommu table/lock into finer graularity pools and lock
>

The poor rds-stress results w/o IOMMU bypass I sent in early post were taken from
kernel that has the above patch and that has all the needed changes in arch/sparc
to use this new feature.

It seems that it worked well for 10G ETH IOMMU lock contention but it still not solving
the rds-stress issue.

The difference can be from:

1. Lock contention still left with this enhancement <-- zero in bypass
2. Overhead to setup the IOMMU mapping <-- almost zero in bypass (require 1 HV call)
3. Overhead to use the IOMMU mapping <-- not sure how to measure this
4. Overhead to tear the IOMMU mapping <-- zero in bypass