Re: [PATCH 03/11] readahead: bump up the default readahead size

[Matt Mackall <mpm@selenic.com>]

On Thu, 2010-02-11 at 23:42 +0000, Jamie Lokier wrote:
> Matt Mackall wrote:
> > On Mon, 2010-02-08 at 21:46 +0800, Wu Fengguang wrote:
> > > Chris,
> > > 
> > > Firstly inform the linux-embedded maintainers :)
> > > 
> > > I think it's a good suggestion to add a config option
> > > (CONFIG_READAHEAD_SIZE). Will update the patch..
> > 
> > I don't have a strong opinion here beyond the nagging feeling that we
> > should be using a per-bdev scaling window scheme rather than something
> > static.
> 
> I agree with both.  100Mb/s isn't typical on little devices, even if a
> fast ATA disk is attached.  I've got something here where the ATA
> interface itself (on a SoC) gets about 10MB/s max when doing nothing
> else, or 4MB/s when talking to the network at the same time.
> It's not a modern design, but you know, it's junk we try to use :-)
> 
> It sounds like a calculation based on throughput and seek time or IOP
> rate, and maybe clamped if memory is small, would be good.
> 
> Is the window size something that could be meaningfully adjusted
> according to live measurements?

I think so. You've basically got a few different things you want to
balance: throughput, latency, and memory pressure. Successful readaheads
expand the window, as do empty request queues, while long request queues
and memory reclaim events collapse it. With any luck, we'll then
automatically do the right thing with fast/slow devices on big/small
boxes with varying load. And, like TCP, we don't need to 'know' anything
about the hardware, except to watch what happens when we use it.

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