From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>,
Ext4 Developers List <linux-ext4@vger.kernel.org>
Subject: Re: Unnecessarily bad cache behavior for ext4_getattr()
Date: Fri, 29 Nov 2019 19:49:50 -0500 [thread overview]
Message-ID: <20191130004950.GB16443@mit.edu> (raw)
In-Reply-To: <CAHk-=wivmk_j6KbTX+Er64mLrG8abXZo0M10PNdAnHc8fWXfsQ@mail.gmail.com>
On Sun, Nov 24, 2019 at 04:19:16PM -0800, Linus Torvalds wrote:
> It looks from profiles like ext4_getattr() is fairly expensive,
> because it unnecessarily accesses the extended inode information and
> causes extra cache misses.
>
> On an empty kernel allmodconfig build (which is a lot of "stat()"
> calls by Make, and a lot of silly string stuff in user space due to
> all the make variable games we play), ext4_getattr() was something
> like 1% of the time according to the profile I gathered. It might be
> bogus - maybe the cacheline ends up being accessed later anyway, but
> it _looked_ like it was the whole "i_extra_isize" access that missed
> in the cache.
>
> That's all for gathering the STATX_BTIME information, that the caller
> doesn't even *want*.
>
> How about a patch like the attached?
Looks good, thanks, I've applied it to the ext4 tree.
I'm a bit surprised a cache line miss rated that high on a kernel
build, but that probably says a lot about how efficient the rest of
the kernel was (and I assume Make didn't need to rebuild most of the
object files).
- Ted
P.S. Did you see the ext4 pull request? I wasn't sure if you haven't
gotten to it yet due to being distracted by Turkey day or not...
prev parent reply other threads:[~2019-11-30 0:49 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-25 0:19 Unnecessarily bad cache behavior for ext4_getattr() Linus Torvalds
2019-11-25 20:35 ` Andreas Dilger
2019-11-30 0:49 ` Theodore Y. Ts'o [this message]
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