linux-kernel - Re: [RFC PATCH linux 2/2] fs/proc: use a hash table for the directory entries

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Message-ID: <20141002200639.GA3497@p183.telecom.by>
Date:	Thu, 2 Oct 2014 23:06:39 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	davem@...emloft.net, akpm@...ux-foundation.org,
	rui.xiang@...wei.com, viro@...iv.linux.org.uk, oleg@...hat.com,
	gorcunov@...nvz.org, kirill.shutemov@...ux.intel.com,
	grant.likely@...retlab.ca, tytso@....edu,
	Thierry Herbelot <thierry.herbelot@...nd.com>
Subject: Re: [RFC PATCH linux 2/2] fs/proc: use a hash table for the
 directory entries

On Thu, Oct 02, 2014 at 11:01:50AM -0700, Eric W. Biederman wrote:
> Nicolas Dichtel <nicolas.dichtel@...nd.com> writes:
> 
> > From: Thierry Herbelot <thierry.herbelot@...nd.com>
> >
> > The current implementation for the directories in /proc is using a single
> > linked list. This is slow when handling directories with large numbers of
> > entries (eg netdevice-related entries when lots of tunnels are opened).
> >
> > This patch enables multiple linked lists. A hash based on the entry name is
> > used to select the linked list for one given entry.
> >
> > The speed creation of netdevices is faster as shorter linked lists must be
> > scanned when adding a new netdevice.
> 
> Is the directory of primary concern /proc/net/dev/snmp6 ?
> 
> Unless I have configured my networking stack weird by mistake that
> is the only directory under /proc/net that grows when we add an
> interface.
> 
> I just want to make certain I am seeing the same things that you are
> seeing.
> 
> I feel silly for overlooking this directory when the rest of the
> scalability work was done.

Slowdown comes from "duplicate name" check:

        for (tmp = dir->subdir; tmp; tmp = tmp->next)
                if (strcmp(tmp->name, dp->name) == 0) {
                        WARN(1, "proc_dir_entry '%s/%s' already registered\n",
                                dir->name, dp->name);
                        break;
                }

Removal can be made O(1) after switching to doubly-linked list.

	Alexey
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