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

[Eric Dumazet]

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 <[EMAIL PROTECTED]>
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.


  




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