Re: [PATCH v5 01/13] mm: Fix the return type of __do_page_cache_readahead

Matthew Wilcox
2020-02-11T12:34:32Z

On Tue, Feb 11, 2020 at 08:19:14AM +0000, Johannes Thumshirn wrote:
> On 11/02/2020 02:05, Matthew Wilcox wrote:
> > even though I'm pretty sure we're not going to readahead more than 2^32
> > pages ever.
>
> And 640K is more memory than anyone will ever need on a computer *scnr*

Sure, but bandwidth just isn't increasing quickly enough to have
this make sense. 2^32 pages even on our smallest page size machines
is 16GB. Right now, we cap readahead at just 256kB. If we did try to
readahead 16GB, we'd be occupying a PCIe gen4 x4 drive for two seconds,
just satisfying this one readahead. PCIe has historically doubled in
bandwidth every three years or so, so to get this down to something
reasonable like a hundredth of a second, we're looking at PCIe gen12 in
twenty years or so. And I bet we still won't do it (also, I doubt PCIe
will continue doubling bandwidth every three years).

And Linus has forbidden individual IOs over 2GB anyway, so not happening
until he's forced to see the error of his ways ;-)