Re: Performance is weird (fwd)

Re: Performance is weird (fwd)

[Manfred Spraul]

One difference between idle and a running user space app is that the
kernel->user space return path checks for pending softirqs, but the ide
thread doesn't.

Perhaps cpu_idle() should also check for pending softirq's before
hlt'ing?

idle thread is running.
* hw interrupt
* * hw interrupt handler
* * * packet arrives
* * * softirq marked
* * hw interrupt handler returns
* do_softirq
* * net_rx called
* * * an hw interrupt interrupts net_rx
* * * * a second packet arrives, softirq marked again.
* * * hw interrupt returns
* * net_rx returns
* do_softirq notices that net_rx is queued again, but doesn't process
    it immediately (otherwise it would cause an endless loop)
* hw interrupt returns
idle thread sleeps again.
!! one packet is waiting unprocessed

What about adding if(softirq_active...) do_softirq() into default_idle?
--
    Manfred

Re: Performance is weird (fwd)

[Manfred Spraul]

[-- Attachment #1: Type: text/plain, Size: 203 bytes --]

I've attached a patch.
I tried to trigger the problem with my 10 MBit ne2k-pci connection, but
without success.

Could you try it?
I've tested it with -ac17, and it applies to 2.4.2 cleanly.

--
	Manfred

[-- Attachment #2: patch-proc --]
[-- Type: text/plain, Size: 399 bytes --]

--- 2.4/arch/i386/kernel/process.c	Thu Feb 22 22:28:52 2001
+++ build-2.4/arch/i386/kernel/process.c	Thu Mar 15 20:35:12 2001
@@ -81,6 +81,11 @@
 {
 	if (current_cpu_data.hlt_works_ok && !hlt_counter) {
 		__cli();
+		if (softirq_active(smp_processor_id()) & softirq_mask(smp_processor_id())) {
+			__sti();
+			do_softirq();
+			return;
+		}
 		if (!current->need_resched)
 			safe_halt();
 		else

Performance is weird (fwd)

[Sampsa Ranta]

--
Subject: Performance is weird
The following message was first posted to linux-atm mailing list, it
is followed with one of the replies I got, thanks Werner Almesberger
<Werner.Almesberger@epfl.ch>.

Actually, with 2.4.3pre4 kernel I got something like 66Mbit/s which were
better than the 2.4.2 results.
--

 Hello,

I am running a set of ForeRunner LE 155 cards on two Athlon 900
machines. The cards are currently back to back connected. I am having
problems with performance and this problem seems a bit curious to me.

The boxes are running kernel versions 2.4.2 with the builtin ATM
functionality.

First when the machine is idle and i run ttcp_atm, the record is:

[root@akvagw test]# ./ttcp_atm -t -a -s 0.90
ttcp-t: buflen=8192, nbuf=2048, align=16384/0, port=5013  atm  -> 0.90
ttcp-t: socket
ttcp-t: 16777216 bytes in 3.805066 real seconds = 4305.838585 KB/sec
(35.273430
Mb/sec)

I can get the same result when I run it as many times as I want when the
machine is idle, however, the performance of the increases a lot when I
give the processor something to do, for example compile the kernel, when
gcc is compiling the kernel, I get better results:

[root@akvagw test]# ./ttcp_atm -t -a -s 0.90
ttcp-t: buflen=8192, nbuf=2048, align=16384/0, port=5013  atm  -> 0.90
ttcp-t: socket
ttcp-t: 16777216 bytes in 0.997561 real seconds = 16424.058278 KB/sec (134.545885 Mb/sec)

For the record, the remote machine does not affect the tests, because the
machine just sends data even when none listens.

Can someone explain, and maybe do something, please? Or am I supposed to
compile kernel all the time on my production ATM routers.

Same seems to apply when I stream UDP via my 3C905C card to one of my
routers, first I get 60Mbytes / s, then 94Mbytes/s when I start to compile
the kernel.

 Thanks,
   Sampsa Ranta
   sampsa@netsonic.fi


"
Don't know where those "negative CPU cycles" come from. It's probably
a driver problem. Could be that either you're triggering scheduling of a
softirq or such, where there normally wouldn't be one (but should be),
or that there's a race condition leading to the loss of an event
(softirq, tasklet, wait queue, etc.), and background activity makes this
happen in the correct order.
"

Re: Performance is weird (fwd)

[Sampsa Ranta]

Hi,

I got my ATM driver working properly, both LE155 and PCA200E did
good throughput when I found out the problem. I had some fancy option in
BIOS setup described like "Enhance chip performance", after turning this
on everything started to rock. So, my best guess is that there was
something terribly wrong in my PCI or chipset setup and this option fixed
the configuration.

My benchmark results on can be found from
http://www.netsonic.fi/~sampsa/netpipe/ if anyone else is interested on
the results. Based on my benchmark results I would say ForeRunner LE155
and PCA-200E and their Linux drivers seemed to benchmark equally well on
throughput, I have no idea on CPU usage, tho.

Thanks,
    Sampsa


> --
> Subject: Performance is weird
> The following message was first posted to linux-atm mailing list, it
> is followed with one of the replies I got, thanks Werner Almesberger
> <Werner.Almesberger@epfl.ch>.
>
> Actually, with 2.4.3pre4 kernel I got something like 66Mbit/s which were
> better than the 2.4.2 results.
> --
>
>  Hello,
>
> I am running a set of ForeRunner LE 155 cards on two Athlon 900
> machines. The cards are currently back to back connected. I am having
> problems with performance and this problem seems a bit curious to me.
>
> The boxes are running kernel versions 2.4.2 with the builtin ATM
> functionality.
>
> First when the machine is idle and i run ttcp_atm, the record is:
>
> [root@akvagw test]# ./ttcp_atm -t -a -s 0.90
> ttcp-t: buflen=8192, nbuf=2048, align=16384/0, port=5013  atm  -> 0.90
> ttcp-t: socket
> ttcp-t: 16777216 bytes in 3.805066 real seconds = 4305.838585 KB/sec
> (35.273430
> Mb/sec)
>
> I can get the same result when I run it as many times as I want when the
> machine is idle, however, the performance of the increases a lot when I
> give the processor something to do, for example compile the kernel, when
> gcc is compiling the kernel, I get better results:
>
> [root@akvagw test]# ./ttcp_atm -t -a -s 0.90
> ttcp-t: buflen=8192, nbuf=2048, align=16384/0, port=5013  atm  -> 0.90
> ttcp-t: socket
> ttcp-t: 16777216 bytes in 0.997561 real seconds = 16424.058278 KB/sec (134.545885 Mb/sec)
>
> For the record, the remote machine does not affect the tests, because the
> machine just sends data even when none listens.
>
> Can someone explain, and maybe do something, please? Or am I supposed to
> compile kernel all the time on my production ATM routers.
>
> Same seems to apply when I stream UDP via my 3C905C card to one of my
> routers, first I get 60Mbytes / s, then 94Mbytes/s when I start to compile
> the kernel.
>
>  Thanks,
>    Sampsa Ranta
>    sampsa@netsonic.fi
>
>
> "
> Don't know where those "negative CPU cycles" come from. It's probably
> a driver problem. Could be that either you're triggering scheduling of a
> softirq or such, where there normally wouldn't be one (but should be),
> or that there's a race condition leading to the loss of an event
> (softirq, tasklet, wait queue, etc.), and background activity makes this
> happen in the correct order.
> "
>
> -
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>