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

[Eric Dumazet <dada1@cosmosbay.com>]

Denys a écrit :
> 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@cosmosbay.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.
>
>
>   




-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/