linux-kernel - Re: spinlock contention of files->file

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Date:	Mon, 30 Sep 2013 19:18:53 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: spinlock contention of files->file_lock

On Mon, 2013-09-30 at 18:44 -0700, Linus Torvalds wrote:
> On Mon, Sep 30, 2013 at 6:05 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > Speaking of spinlock contention, the files->file_lock is a good example.
> >
> > Multi threaded programs hit this spinlock three times per fd :
> 
> .. do you actually have programs that see contention?

Yes, Google-Bug-Id 9072743 for Googlers ;)

Basically, we have (many) servers with 10,000,000+ sockets, where this
lock contention is a real problem.

We use a SOCK_FD_FASTALLOC accept4()/socket() flag to relax POSIX
requirements and not having to parse huge bitmap in
find_next_zero_bit(fdt->open_fds, fdt->max_fds, fd)  (O_FD_FASTALLOC
uses a random starting point)

I am writing a patch using cmpxchg() to see if I can solve the dup2()
'problem'


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