SD card performace ?
sears at cs.berkeley.edu
Mon Aug 4 20:43:51 CEST 2008
Andy Green wrote:
> | is this really the max. cpu/system performace for a 400 MHz arm ?
> No, it's not CPU bound. For "half the time" the CPU is doing other
> processes waiting for the bulk completion interrupt from Glamo.
Wait, writes go into a buffer after being ECC'ed and compressed, but
before being sent to the sd device, right?
The usual rule of thumb is one outstanding write per wire. So for a
desktop computer, you need might need to maintain three outstanding
writes: one for the PCI bus, one for the sata cable, and one for the
If we have such a buffer, one page in the buffer is used for s/w ecc,
compression, etc, and one for the card to write data from. This
overlaps SD write time with CPU mucking with data time, so the total
time should be "max(CPU time, SD time)", not "CPU time + SD time", and
software ECC shouldn't affect read performance, unless the cpu is at
100% during the test.
If we're overlapping I/O and CPU, and the CPU is not at 100%, shouldn't
the I/O bandwidth be the minimum of the bus bandwidth and the sd card
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