Re: [PATCH] Avoiding fragmentation through different allocator

Andi Kleen
2005-01-25T14:49:35Z

On Tue, Jan 25, 2005 at 02:27:57PM +0000, Christoph Hellwig wrote:
> > It is not the driver per se, but the way the memory which is the I/O
> > source/target is presented to the driver. In linux there is a good
> > chance it will have to use more scatter gather elements to represent
> > the same amount of data.
>
> Note that a change made a few month ago after seeing issues with
> aacraid means it's much more likely to see contingous memory,
> there were some numbers on linux-scsi and/or linux-kernel.

But only at the beginning. iirc after a few days of uptime
and memory fragmentation it degenerates back to the old numbers.

Perhaps the recent anti defragmentation work will help more.

-Andi

P.S.: on a AMD x86-64 box the theory can be relatively easily tested:
just run with iommu=force,biomerge that will use the IOMMU to merge
SG elements. I just don't recommend it for production because some errors
are not well handled.