Re: [PATCH] speed up SATA

[Jeff Garzik <jgarzik@pobox.com>]

Marc Bevand wrote:
> 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.

[...]

Very cool, thanks for benching!


> 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.

This is not surprising, since:
* the scatter-gather table imposes a limit of 8MB
* the VM further imposes limits on readahead and writeback

So 32MB is the hardware max, but not really achieveable due to other 
factors.


> 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.

Yeah, this is only found in the bleeding-edge-latest 2.6.x kernels.  You 
did the right thing...

Regards,

	Jeff