netdev - Re: 2.6.21 -> 2.6.22 & 2.6.23-rc8 performance regression

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Date:	Mon, 1 Oct 2007 15:10:15 +0300
From:	"Denys" <nuclearcat@...learcat.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: 2.6.21 -> 2.6.22 & 2.6.23-rc8 performance regression

Not able to compile kernel with patch

drivers/built-in.o: In function `secure_tcp_sequence_number':
(.text+0x3ad02): undefined reference to `__divdi3'
make: *** [.tmp_vmlinux1] Error 1

On Mon, 01 Oct 2007 10:20:07 +0200, Eric Dumazet wrote
> Denys a :
> > Well, i can play a bit more on "live" servers. I have now hot-swap server with
> > full gentoo,  where i can rebuild any kernel you want, with any applied patch.
> > But it looks more like not overhead, load becoming high too "spiky", and it is
> > not just permantenly higher. Also it is not normal that all system becoming
> > unresposive (for example ping 127.0.0.1 becoming 300ms for period, when usage
> > softirq jumps to 100%).
> >
> >   
> Could you try a pristine 2.6.22.9 and some patch in 
> secure_tcp_sequence_number() like :
> 
> --- drivers/char/random.c.orig 2007-10-01 10:18:42.000000000 +0200
> +++ drivers/char/random.c 2007-10-01 10:19:58.000000000 +0200
> @@ -1554,7 +1554,7 @@
> * That's funny, Linux has one built in! Use it!
> * (Networks are faster now - should this be increased?)
> */
> - seq += ktime_get_real().tv64;
> + seq += ktime_get_real().tv64 / 1000;
> #if 0
> printk("init_seq(%lx, %lx, %d, %d) = %d\n",
> saddr, daddr, sport, dport, seq);
> 
> Thank you
> 
> > On Mon, 01 Oct 2007 00:12:59 -0700 (PDT), David Miller wrote
> >   
> >> From: Eric Dumazet <dada1@...mosbay.com>
> >> Date: Mon, 01 Oct 2007 07:59:12 +0200
> >>
> >>     
> >>> No problem here on bigger servers, so I CC David Miller and netdev
> >>> on this one.  AFAIK do_gettimeofday() and ktime_get_real() should
> >>> use the same underlying hardware functions on PC and no performance
> >>> problem should happen here.
> >>>       
> >> One thing that jumps out at me is that on 32-bit (and to a certain
> >> extent on 64-bit) there is a lot of stack accesses and missed
> >> optimizations because all of the work occurs, and gets expanded,
> >> inside of ktime_get_real().
> >>
> >> The timespec_to_ktime() inside of there constructs the ktime_t return
> >> value on the stack, then returns that as an aggregate to the caller.
> >>
> >> That cannot be without some cost.
> >>
> >> ktime_get_real() is definitely a candidate for inlining especially in
> >> these kinds of cases where we'll happily get computations in local
> >> registers instead of all of this on-stack nonsense.  And in several
> >> cases (if the caller only needs the tv_sec value, for example)
> >> computations can be elided entirely.
> >>
> >> It would be constructive to experiment and see if this is in fact 
> >> part of the problem.
> >>     
> >
> >
> > --
> > Denys Fedoryshchenko
> > Technical Manager
> > Virtual ISP S.A.L.
> >
> >
> >


--
Denys Fedoryshchenko
Technical Manager
Virtual ISP S.A.L.

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