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RE: Getting/Forcing Greater than 4KB Buffer Allocations



> On Tue, Jul 17, 2007 at 04:54:31PM -0700, David Christensen wrote:
>  > I'm investigating a problem with my bce driver which 
> occurs when I ask
>  > for a jumbo
>  > mbuf cluster (through m_cljget()).  When I map the memory for DMA I
>  > normally 
>  > get 3 memory segments (4KB + 4KB + 1KB) on my system, but 
> on another
>  > user's 
>  > system he's seeing 2 memory segments (8KB + 1KB).  Is there a
>  > configuration
>  > option that allows this or some other tuning variable 
> involved?  The
>  > system is a 
>  > Xeon dual-core processor and has 8GB of RAM, running an 
> AMD64 version of
>  > the kernel.
>  >  
> 
> I've briefly looked over bus_dma usage on bce(4). It seems that you
> told bus_dma the the dma map could be made up of BCE_MAX_SEGMENTS
> segments, where a dma segment could be MJUM9BYTES bytes. If you want
> just two segments you may have to use 2 instead of BCE_MAX_SEGMENTS.
> If the hardware can support up to BCE_MAX_SEGMENTS dma segments on Rx
> descriptors you should be prepared to handle that number of dma
> segments too(e.g. You don't know how may dma segments would be
> returned by bus_dma, you just know the upper bound as you specified
> in bus_dma_tag_create()).
> If the hardware can handle just up to 4KB for a dma segment you
> should tell bus_dma the restriction of the dma segment.
> If you have to get a single dma segment that covers MJUM9BYTES bytes
> due to the limitation of the hardware you may have to use local
> allocator.
> 

Thanks Pyun but I'm really just looking for a way to test that I can
handle the number of segments I've advertised that I can support.  I 
believe my code is correct but when all I see are allocations of 3 
segments I just can't prove it.  I was hoping that running a utility
such as "stress" would help fragment memory and force more variable
responses but that hasn't happened yet.

Dave

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