Subject: Re: [PATCH] /proc/net/tcp, overhead removed

From: Stephen Hemminger <[email-masked]>
Date: Mon, 28 Sep 2009 16:24:17 -0700

On Tue, 29 Sep 2009 00:20:07 +0200
Eric Dumazet <[email-masked]> wrote:

> Yakov Lerner a écrit :
> > On Sun, Sep 27, 2009 at 12:53, Eric Dumazet <[email-masked]> wrote:
> >> Yakov Lerner a écrit :
> >>> /proc/net/tcp does 20,000 sockets in 60-80 milliseconds, with this patch.
> >>>
> >>> The overhead was in tcp_seq_start(). See analysis (3) below.
> >>> The patch is against Linus git tree (1). The patch is small.
> >>>
> >>> ------------  -----------   ------------------------------------
> >>> Before patch  After patch   20,000 sockets (10,000 tw + 10,000 estab)(2)
> >>> ------------  -----------   ------------------------------------
> >>> 6 sec          0.06 sec     dd bs=1k if=/proc/net/tcp >/dev/null
> >>> 1.5 sec        0.06 sec     dd bs=4k if=/proc/net/tcp >/dev/null
> >>>
> >>> 1.9 sec        0.16 sec     netstat -4ant >/dev/null
> >>> ------------  -----------   ------------------------------------
> >>>
> >>> This is ~ x25 improvement.
> >>> The new time is not dependent on read blockize.
> >>> Speed of netstat, naturally, improves, too; both -4 and -6.
> >>> /proc/net/tcp6 does 20,000 sockets in 100 millisec.
> >>>
> >>> (1) against git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git
> >>>
> >>> (2) Used 'manysock' utility to stress system with large number of sockets:
> >>>   "manysock 10000 10000"    - 10,000 tw + 10,000 estab ip4 sockets.
> >>>   "manysock -6 10000 10000" - 10,000 tw + 10,000 estab ip6 sockets.
> >>> Found at http://ilerner.3b1.org/manysock/manysock.c
> >>>
> >>> (3) Algorithmic analysis.
> >>>     Old algorithm.
> >>>
> >>> During 'cat </proc/net/tcp', tcp_seq_start() is called O(numsockets) times (4).
> >>> On average, every call to tcp_seq_start() scans half the whole hashtable. Ouch.
> >>> This is O(numsockets * hashsize). 95-99% of 'cat </proc/net/tcp' is spent in
> >>> tcp_seq_start()->tcp_get_idx. This overhead is eliminated by new algorithm,
> >>> which is O(numsockets + hashsize).
> >>>
> >>>     New algorithm.
> >>>
> >>> New algorithms is O(numsockets + hashsize). We jump to the right
> >>> hash bucket in tcp_seq_start(), without scanning half the hash.
> >>> To jump right to the hash bucket corresponding to *pos in tcp_seq_start(),
> >>> we reuse three pieces of state (st->num, st->bucket, st->sbucket)
> >>> as follows:
> >>>  - we check that requested pos >= last seen pos (st->num), the typical case.
> >>>  - if so, we jump to bucket st->bucket
> >>>  - to arrive to the right item after beginning of st->bucket, we
> >>> keep in st->sbucket the position corresponding to the beginning of
> >>> bucket.
> >>>
> >>> (4) Explanation of O( numsockets * hashsize) of old algorithm.
> >>>
> >>> tcp_seq_start() is called once for every ~7 lines of netstat output
> >>> if readsize is 1kb, or once for every ~28 lines if readsize >= 4kb.
> >>> Since record length of /proc/net/tcp records is 150 bytes, formula for
> >>> number of calls to tcp_seq_start() is
> >>>             (numsockets * 150 / min(4096,readsize)).
> >>> Netstat uses 4kb readsize (newer versions), or 1kb (older versions).
> >>> Note that speed of old algorithm does not improve above 4kb blocksize.
> >>>
> >>> Speed of the new algorithm does not depend on blocksize.
> >>>
> >>> Speed of the new algorithm does not perceptibly depend on hashsize (which
> >>> depends on ramsize). Speed of old algorithm drops with bigger hashsize.
> >>>
> >>> (5) Reporting order.
> >>>
> >>> Reporting order is exactly same as before if hash does not change underfoot.
> >>> When hash elements come and go during report, reporting order will be
> >>> same as that of tcpdiag.
> >>>
> >>> Signed-off-by: Yakov Lerner <[email-masked]>

Does the netlink interface used by ss command have the problem?

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