2.6.21 -> 2.6.22 & 2.6.23-rc8 performance regression

["Denys" <nuclearcat@nuclearcat.com>]

I've moved recently one of my proxies(squid and some compressing application) 
from 2.6.21 to 2.6.22, and notice huge performance drop. I think this is 
important, cause it can cause serious regression on some other workloads like 
busy web-servers and etc.

After some analysis of different options i can bring more exact numbers:

2.6.21 able to process 500-550 requests/second and 15-20 Mbit/s of traffic, 
and working great without any slowdown or instability.

2.6.22 able to process only 250-300 requests and 8-10 Mbit/s of traffic, ssh 
and console is "freezing" (there is delay even for typing characters).

Both proxies is on identical hardware(Sun Fire X4100), configuration(small 
system, LFS-like, on USB flash), different only kernel.

I tried to disable/enable various options and optimisations - it doesn't 
change anything, till i reach SLUB/SLAB option.

I've loaded proxy configuration to gentoo PC with 2.6.22 (then upgraded it to 
2.6.23-rc8), and having same effect.
Additionally, when load reaching maximum i can notice whole system slowdown, 
for example ssh and scp takes much more time to run, even i do nice -n -5 for 
them.

But even choosing 2.6.23-rc8+SLAB i noticed same "freezing" of ssh (and sure 
it slowdown other kind of network performance), but much less comparing with 
SLUB. On top i am seeing ksoftirqd taking almost 100% (sometimes ksoftirqd/0, 
sometimes ksoftirqd/1).

I tried also different tricks with scheduler (/proc/sys/kernel/sched*), but 
it's also didn't help.

When it freezes it looks like:
  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
    7 root      15  -5     0    0    0 R   64  0.0   2:47.48 ksoftirqd/1
 5819 root      20   0  134m 130m  596 R   57  3.3   4:36.78 globax
 5911 squid     20   0 1138m 1.1g 2124 R   26 28.9   2:24.87 squid
   10 root      15  -5     0    0    0 S    1  0.0   0:01.86 events/1
 6130 root      20   0  3960 2416 1592 S    0  0.1   0:08.02 oprofiled


Oprofile results:


Thats oprofile with 2.6.23-rc8 - SLUB

73918    21.5521  check_bytes
38361    11.1848  acpi_pm_read
14077     4.1044  init_object
13632     3.9747  ip_send_reply
8486      2.4742  __slab_alloc
7199      2.0990  nf_iterate
6718      1.9588  page_address
6716      1.9582  tcp_v4_rcv
6425      1.8733  __slab_free
5604      1.6339  on_freelist


Thats oprofile with 2.6.23-rc8 - SLAB

CPU: AMD64 processors, speed 2592.64 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a unit 
mask of 0x00 (No unit mask) count 100000
samples  %        symbol name
138991   14.0627  acpi_pm_read
52401     5.3018  tcp_v4_rcv
48466     4.9037  nf_iterate
38043     3.8491  __slab_alloc
34155     3.4557  ip_send_reply
20963     2.1210  ip_rcv
19475     1.9704  csum_partial
19084     1.9309  kfree
17434     1.7639  ip_output
17278     1.7481  netif_receive_skb
15248     1.5428  nf_hook_slow

My .config is at http://www.nuclearcat.com/.config (there is SPARSEMEM 
enabled, it doesn't make any noticeable difference)

Please CC me on reply, i am not in list.

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