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SubjectRe: 2.6.21 -> 2.6.22 & 2.6.23-rc8 performance regression
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Hi Nick

There is no numbers for sure now, it is just instability.

I am bisecting kernel now (from 2.6.21 to 2.6.22-rc1), i think it is caused
by some patch, and maybe not related to CFS. Anyways, let's see when i finish
bisect. Just i was thinking, maybe anyone else have similar issues or some
ideas in mind.

The only one case - it is very difficult to track, and i have to do it on
live server with thousands of users. At night load is much lower, so possible
by mistake i will miss a point (it can detect bug easily, IF softirq in
mpstat 1 will jump up to 30+%. On 2.6.21 it is always staying +-10%, on
"buggy" version it jumps up to 100%. Now cause less load at night up to 30-
50%, so on hardware it is not very noticeable (thats why maybe not much
feedbacks), i am trying to utilise hardware maximum as possible - so i feel
regressions much more.

Anyway, possible i will have news after 2-3 hours max.


On Sun, 30 Sep 2007 14:25:37 +1000, Nick Piggin wrote
> Hi Denys, thanks for reporting (btw. please reply-to-all when
> replying on lkml).
>
> You say that SLAB is better than SLUB on an otherwise identical
> kernel, but I didn't see if you quantified the actual numbers? It
> sounds like there is still a regression with SLAB?
>
> On Monday 01 October 2007 03:48, Eric Dumazet wrote:
> > Denys a :
> > > 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.
> >
> > Could you try with SLUB but disabling CONFIG_SLUB_DEBUG ?


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

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