Linus Still Based and Caches Are Faster than Direct IO

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Linux Torvalds has been known to be a tad harsh when it has been required to stop gross incompetence from ruining the Linux Kernel. Many feared that the introduction of a CoC in Kernel 4.19 and the forced re-education camp he was subjected to had changed Linus' ability to firmly reject undesirable pieces of code. It would appear that his ability to clearly explain fundamental computing concepts when it is needed is very much intact.

Linus Torvalds Otaniemi June 14, 2012.jpg

From Linus Torvalds <>
Date Thu, 13 Jun 2019 16:30:36 -1000
Subject Re: pagecache locking (was: bcachefs status update) merged)
On Thu, Jun 13, 2019 at 1:56 PM Dave Chinner <david@fromorbit.com> wrote:
>
> That said, the page cache is still far, far slower than direct IO,
Bullshit, Dave.
You've made that claim before, and it's been complete bullshit before
too, and I've called you out on it then too.
Why do you continue to make this obviously garbage argument?
The key word in the "page cache" name is "cache".
Caches work, Dave. Anybody who thinks caches don't work is
incompetent. 99% of all filesystem accesses are cached, and they never
do any IO at all, and the page cache handles them beautifully.
When you say that the page cache is slower than direct IO, it's
because you don't even see or care about the *fast* case. You only get
involved once there is actual IO to be done.
So you're making that statement without taking into account all the
cases that you don't see, and that you don't care about, because the
page cache has already handled them for you, and done so much better
than DIO can do or ever _will_ do.
Is direct IO faster when you *know* it's not cached, and shouldn't be
cached? Sure. But that/s actually quite rare.
How often do you use non-temporal stores when you do non-IO
programming? Approximately never, perhaps? Because caches work.
And no, SSD's haven't made caches irrelevant. Not doing IO at all is
still orders of magnitude faster than doing IO. And it's not clear
nvdimms will either.
So stop with the stupid and dishonest argument already, where you
ignore the effects of caching.

It would appear that Linus Torvalds is still very based.

We'll just leave the first half of Dave's reply for your enjoyment below.

Date Fri, 14 Jun 2019 17:30:53 +1000
From Dave Chinner <>
Subject Re: pagecache locking (was: bcachefs status update) merged)
On Thu, Jun 13, 2019 at 04:30:36PM -1000, Linus Torvalds wrote:
> On Thu, Jun 13, 2019 at 1:56 PM Dave Chinner <david@fromorbit.com> wrote:
> >
> > That said, the page cache is still far, far slower than direct IO,
>
> Bullshit, Dave.
>
> You've made that claim before, and it's been complete bullshit before
> too, and I've called you out on it then too.
Yes, your last run of insulting rants on this topic resulted in me
pointing out your CoC violations because you were unable to listen
or discuss the subject matter in a civil manner. And you've started
right where you left off last time....

The full e-mail exchange can be enjoyed at https://lkml.org/lkml/2019/6/13/1892


published 2019-06-20last edited 2019-06-20


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