[syslinux] Slow performance booting USB device with 3.xx version

[H. Peter Anvin]

Andrea Mazzoleni wrote:
> With the new 3.01 and 3.07 version I found a big slowdown booting from
> USB devices.
> 
> On my PC it takes more than 2 minutes to load the kernel and the
> initrd filesystem. Compared with the previous behaviour of syslinux
> 2.13 with takes only 10 seconds or little more.
> 
> The BIOS boots in FDD mode (DriveNumber variable is 0). I've also
> tried to disable the EDD support in the source. But the boot process
> takes always a long time.
> 
> I've also a PC which boots in HDD mode (DriveNumber has the highest
> bit set) and it is also slower, approx 20 seconds compared to the
> previous 10.
> 
> I don't understand where this slowdown come from. Which changed
> exactly from the 2.13 version ? Peter, do you have some hints for my
> tests ?
> 
> I'm very interested in the new FAT32 support, but I don't like to have
> regression from the previous version.
> 

The two differences are EDD support, and the fact that the FAT isn't all 
sucked in before the command prompt is shown.  Instead there is a 64K 
cache used for the FAT and directory information.

This should pretty much amount to an extra access every 256 sectors (for 
FAT16), but I have a very hard time imagining the construction of a USB 
BIOS which would suffer that greatly.

The fact that the BIOS boots in floppy mode is an indication that 
something is very wrong to begin with.  Have you managed to get it into 
HDD mode, or at least zip mode?  If you're in floppy mode, what's the 
geometry?

It's possible that you're regressing into sector-at-a-time mode, but 
that's very hard for me to know without access to the particular system 
that you're on.

	-hpa