2.2.19pre3 and poor reponse to RT-scheduled processes?

2.2.19pre3 and poor reponse to RT-scheduled processes?

[Rafal Boni]

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[...Please CC me on any replies, as I'm not on the list(s)...]

Folks:
	I was experiencing problems with 2.2.16 where the box would go out
	to lunch for a few seconds flushing buffer or paging at inopportune
	times (is there ever an opportune time for the box to become non-
	reponsive for 5 seconds? 8-).

	2.2.19pre3 makes the behaviour much better, but I still see ~ 2sec
	pauses at times.  I'm sending this to the MM list as well, since I
	believe the poor behaviour in 2.2.16 was an MM issue... I don't 
	know where the slowdowns are happening this time around.

	The box in question is running the linux-ha.org heartbeat package,
	which is a RT-scheduled, mlock()'ed process, and as such should
	get as good service as the box is able to mange.  Often, under
	high disk (and/or MM) loads, the box becomes unreponsive for a
	period of time from ~ 1 sec to a high of ~ 2.8sec.

	The test is simply running a 'dd if=/dev/zero of=/u1/big-empty-file
	bs=1k count=512000 && date'.  Generally, the box will sieze up around
	the same time as the the 'dd' finishes (maybe trying to exec date?).

	I'd appreciate any hints at how to reduce the non-reponsiveness 
	window down as much as possible.  I haven't yet looked to see if
	there is a version of the low-latency patches for 2.2.18 or 19pre,
	but I'd appreciate other ideas on tracking this down as well.

Thanks!
- --rafal

- ----
Rafal Boni                                               rafal.boni@eDial.com
 PGP key C7D3024C, print EA49 160D F5E4 C46A 9E91  524E 11E0 7133 C7D3 024C
    Need to get a hold of me?  http://800.eDial.com/rafal.boni@eDial.com

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Re: 2.2.19pre3 and poor reponse to RT-scheduled processes?

[Gregory Maxwell]

On Fri, Dec 29, 2000 at 03:45:23PM -0500, Rafal Boni wrote:
[snip]
> 	The box in question is running the linux-ha.org heartbeat package,
> 	which is a RT-scheduled, mlock()'ed process, and as such should
> 	get as good service as the box is able to mange.  Often, under
> 	high disk (and/or MM) loads, the box becomes unreponsive for a
> 	period of time from ~ 1 sec to a high of ~ 2.8sec.
[snip]

You are running IDE aren't you?

Enable DMA and/or unmask interupts.

man hdparm

Good luck.
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Re: 2.2.19pre3 and poor reponse to RT-scheduled processes?

[Rafal Boni]

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In message <20001229161927.A560@xi.linuxpower.cx>, Greg Maxwell wrote:

- -> You are running IDE aren't you?
- -> 
- -> Enable DMA and/or unmask interupts.

D'oh!  Thanks to Greg for the clue-by-four!  I *am* running IDE and I had
both DMA (due to misreading of kernel boot message) and interrupt unmasking 
(since I had forgotten that one) off....

I had assumed that DMA was on from the mention of it in kernel messages 
(which on closer reading do indicate CMOS/BIOS configured default modes,
not what the kernel is using), and the lack of an explicit message on
the order of "I know it's there, but I'm not going to use it all the
same" 8-)

Now my box behaves much more reasonably... I'll just have to beat harder
on it and see what happens.

Thank for the help,
- --rafal

- ----
Rafal Boni                                              rafal.boni@eDial.com
 PGP key C7D3024C, print EA49 160D F5E4 C46A 9E91  524E 11E0 7133 C7D3 024C
    Need to get a hold of me?  http://800.edial.com/rafal.boni@eDial.com

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Re: 2.2.19pre3 and poor reponse to RT-scheduled processes?

[Andrea Arcangeli]

On Fri, Dec 29, 2000 at 04:54:23PM -0500, Rafal Boni wrote:
> Now my box behaves much more reasonably... I'll just have to beat harder
> on it and see what happens.

Another thing: while writing to disk if you want low latency readers you can
do:

	elvtune -r 1 /dev/hd[abcd]

The 1/2 seconds stalls you see could be just because of applications that waits
I/O synchronously while the elevator is reodering I/O requests (and even if the
elevator wouldn't reorder anything the new requests would go to the end of the
I/O queue so they would have some higher latency anyways). That's normal and if
it's the case to avoid those stalls you can only decrease the I/O load or
increase disk throughput ;). The important thing is that the kernel is
not sitting in a tight kernel loop without reschedule in it during such 2
seconds.

However 2.2.19pre3aa4 includes also the lowlatency bugfixes in case you have
tons of ram and you're sending huge buffers to syscalls.

Andrea
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