Re: [PATCH] speed up SATA

[Marc Bevand <bevand_m@epita.fr>]

Jeff Garzik wrote:
> 
> [...]
> With this simple patch, the max request size goes from 128K to 32MB... 
> so you can imagine this will definitely help performance.  Throughput 
> goes up.  Interrupts go down.  Fun for the whole family.
> [...]

I have experienced a noticeable improvement concerning the CPU usage
and disk throughput with this patch.

Benchmark specs:

  o read from only 1 disk (sda), or from 2 disks (sda+sdb), with
    1 or 2 instances of "dd if=/dev/sd? of=/dev/null bs=100M".
  o hardware: two Seagate 160GB SATA, on a Silicon Image 3114, on a
    32-bit/33MHz PCI bus, 1GB RAM.
  o software: kernel 2.6.5-rc2-bk6-libata2.

Benchmark datas:

     without the speed-up-sata patch       with the speed-up-sata patch
     reading sda     reading sda+sdb     reading sda     reading sda+sdb
bi      57000             92000             57000             97000
in       1900              2400              1600              1800
cs       1800              3300              1400              1700
sy        11%               20%                9%               16%
us         0%                0%                0%                0%

("bi, in, cs, sy, us" have been reported by vmstat(8))

When reading only from sda, the speed-up-sata patch makes the number of
interrupts/s drop from 1900 to 1600 (CPU usage: 11% to 9%). The throughput
does not improve because 57000 blocks/s is the physical limit of the hardisk.

When reading from both sda and sdb, the improvement is more visible: the
number of interrupts/s goes from 2400 to 1800 (CPU usage: 20% to 16%). But
in this case, the throughput improves from 92000 blocks/s to 97000 blocks/s.
I think I am reaching the physical limit of the PCI bus (theoretically it
would be 133 MB/s or 133000 blocks/s). When setting the PCI latency timer of
the SiI3114 controller to 240 (was 64), I am able to reach 100000 blocks/s.

As other people were complaining that the 32MB max request size might be too
high, I did give a try to 1MB (by replacing "65534" by "2046" in the patch).
There is no visible differences between 32MB and 1MB.

PS: Jeff: "pci_dma_mapping_error()", in libata-core.c from your latest
2.6-libata patch, is an unresolved symbol. I have had to comment it out
to be able to compile the kernel.

-- 
Marc Bevand