Douglas Gilbert wrote:
> Alan Cox wrote:
> >
> > scsi0: scanning channel 1 for devices.
> > scsi0: scanning virtual channel for logical drives.
> > Vendor: MegaRAID Model: LD0 RAID5 26094R Rev: 3.00
> > Type: Direct-Access ANSI SCSI revision: 02
> > Detected scsi disk sda at scsi0, channel 1, id 0, lun 0
> > scsi::resize_dma_pool: WARNING, dma_sectors=0, wanted=13200, scaling
> > scsi::resize_dma_pool: WARNING, dma_sectors=0, wanted=9904, scaling
> > scsi::resize_dma_pool: WARNING, dma_sectors=0, wanted=7440, scaling
> > scsi::resize_dma_pool: WARNING, dma_sectors=0, wanted=5584, scaling
> > SCSI device sda: hdwr sector= 512 bytes. Sectors= 53440512 [26094 MB] [26.1 GB]
>
> The dma_pool in the scsi mid level still makes the worst case
> assumption and allocates memory with the GFP_DMA flag (line
> 2627 of scsi.c in 2.3.40). On the i386 this implies memory
> must be found in the first 16 MB. The first request was for
> about 6.5 MB and the kernel couldn't find enough pages, so
> those were freed up and it asked for 5 MB and the kernel
> ... etc. This algorithm is an alternative to panicing or
> ignoring a NULL result from kmalloc().
>
> Does the MegaRAID really need 6.5 MBytes of scsi dma buffers?
> Has the zoning allocator been changed making memory under
> 16 MB harder to find?
I was a bit surprised that the amount of DMA memory was really this high. I ran
the numbers
and based upon what I saw in the host template 13200 is correct. Worse yet, the
overallocation
that I talked about before doesn't enter into the picture. This card is faking a
single device.
The first question is whether it is in fact correct that this card uses ISA
DMA. If in fact this flag
is set incorrectly it would drastically reduce the size of the DMA pool needed for
megaraid. My
reading of the sources indicates that this is a PCI device at least in some if not
all cases, and that
is inconsistent (I would hope) with having unchecked_isa_dma set to 1.
Let's just say that if this was set incorrectly, clearing that bit would reduce
the DMA pool requirements
for this device from about 6Mb to about 50Kb. Performance would be much better too
as there wouldn't
be a need for bounce buffers.
-Eric
--
"The world was a library, and its books were the stones, leaves,
brooks, grass, and the birds of the earth. We learned to do what only
a student of nature ever learns, and that was to feel beauty."
Chief Luther Standing Bear - Teton Sioux
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