From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
"Julien Grall" <julien@xen.org>, "Wei Liu" <wl@xen.org>,
Xen-devel <xen-devel@lists.xenproject.org>,
"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH] xen/list: Remove prefetching
Date: Wed, 15 Jan 2020 12:17:42 +0100 [thread overview]
Message-ID: <ad6edefe-a295-d00b-179b-c95efc5f34b0@suse.com> (raw)
In-Reply-To: <20200114203545.8897-1-andrew.cooper3@citrix.com>
On 14.01.2020 21:35, Andrew Cooper wrote:
> Xen inherited its list infrastructure from Linux. One area where has fallen
> behind is that of prefetching, which as it turns out is a performance penalty
> in most cases.
>
> Prefetch of NULL on x86 is now widely measured to have glacial performance
> properties, and will unconditionally hit on every hlist use due to the
> termination condition.
>
> Cross-port the following Linux patches:
>
> 75d65a425c (2011) "hlist: remove software prefetching in hlist iterators"
> e66eed651f (2011) "list: remove prefetching from regular list iterators"
> c0d15cc7ee (2013) "linked-list: Remove __list_for_each"
Just as an observation (not an objection), the 2nd of these says
"normally the downsides are bigger than the upsides", which makes
it unbelievably clear what these supposed downsides are. I can
accept prefetches through NULL to be harmful. I can also accept
prefetches on single entry lists to not be very useful. But does
this also render them useless on long lists with not overly much
cache churn done by the body of the iteration loop? Wouldn't it
at least be worthwhile to have list_for_each_prefetch() retaining
prior behavior, and use it in places where prefetching can be
deemed to help?
Jan
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel
next prev parent reply other threads:[~2020-01-15 11:18 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-14 20:35 [Xen-devel] [PATCH] xen/list: Remove prefetching Andrew Cooper
2020-01-14 20:58 ` Julien Grall
2020-01-15 10:39 ` Roger Pau Monné
2020-01-15 11:25 ` Andrew Cooper
2020-01-15 11:17 ` Jan Beulich [this message]
2020-01-15 12:40 ` Andrew Cooper
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ad6edefe-a295-d00b-179b-c95efc5f34b0@suse.com \
--to=jbeulich@suse.com \
--cc=Volodymyr_Babchuk@epam.com \
--cc=andrew.cooper3@citrix.com \
--cc=julien@xen.org \
--cc=roger.pau@citrix.com \
--cc=sstabellini@kernel.org \
--cc=wl@xen.org \
--cc=xen-devel@lists.xenproject.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.