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Date: Tue, 26 Aug 2008 20:32:31 +0200 From: Eric Dumazet <dada1@...mosbay.com> To: Dave Jones <davej@...hat.com> Cc: netdev@...r.kernel.org, j.w.r.degoede@....nl Subject: Re: cat /proc/net/tcp takes 0.5 seconds on x86_64 Dave Jones a écrit : > Just had this bug reported against our development tree.. > > Dave > > On Tue, Aug 26, 2008 at 11:49:31AM -0400, bugzilla@...hat.com wrote: > > Please do not reply directly to this email. All additional > > comments should be made in the comments box of this bug. > > > > https://bugzilla.redhat.com/show_bug.cgi?id=459782 > > > > Hans de Goede <j.w.r.degoede@....nl> changed: > > > > What |Removed |Added > > ---------------------------------------------------------------------------- > > CC| |j.w.r.degoede@....nl > > Component|gkrellm |kernel > > AssignedTo|j.w.r.degoede@....nl |kernel-maint@...hat.com > > Summary|gkrellmd consumes about 75% |cat /proc/net/tcp takes 0.5 > > |cpu time |seconds on x86_64, 0.5 > > | |seconds !! > > > > --- Comment #2 from Hans de Goede <j.w.r.degoede@....nl> 2008-08-26 11:49:30 EDT --- > > Thanks for reporting this some stracing of gkrellmd has found that reading from > > /proc/net/tcp and reading from /proc/net/tcp6 is the culprit, try this on your > > x86_64 machine to confirm: > > > > "time cat /proc/net/tcp" > > > > To give you an idea on my rawhide x86_64 machine: > > [hans@...alhost devel]$ time cat /proc/net/tcp > > <snip> > > real 0m0.520s > > user 0m0.000s > > sys 0m0.446s > > > > Thats amazingly slow, esp as I only have 8 tcp connections open. > > > > Some maybe usefull info: top reports a very high load (50%) from soft IRQ's. > > > > Anyways changing this to a kernel bug. > I wonder why this qualifies as a "kernel bug". This is a well known problem. At least, current kernel versions no longer block softirq for long periods while doing this... cat /proc/net/tcp is slow and deprecated, since it uses a O(N^2) algo. tcp hash table size might be a litle bit too large for typical setups (few tcp session, even on a 16 GB machine) Unfortunatly it is fixed at boot time and not dynamic (yet) You can : 1) Boot your machine with a boot cmd "thash_entries=1024" to reduce size of TCP hash table (typical size on a 4GB machine is : TCP established hash table entries: 262144 (order: 10, 4194304 bytes)) (max size is 524288 entries for machines with >= 8GB memory if no "thash_entries=..." specified, since October 2007 see http://kerneltrap.org/mailarchive/linux-netdev/2007/10/26/359198 ) 2) Switch to netlink interface instead of /proc/net/tcp[6] legacy file. Example : netstat -N http://www.ducksong.com/misc/netstat-netlink-diag-patch.txt 3) Use both 1) & 2) :) 4) Submit a patch to dynamically grow tcp hash table :) Links: http://kerneltrap.org/mailarchive/linux-netdev/2007/11/1/375782 http://kerneltrap.org/mailarchive/linux-netdev/2007/11/1/376907 Time difference between /proc/net/tcp and netlink on a 4GB x86_64 machine : # dmesg | grep "TCP established hash" TCP established hash table entries: 262144 (order: 10, 4194304 bytes) # time cat /proc/net/tcp >/dev/null real 0m0.091s user 0m0.001s sys 0m0.090s # time ss -n >/dev/null # ss uses netlink interface real 0m0.022s user 0m0.000s sys 0m0.022s -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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