Re: Apparent strange disk behaviour in 6.0

From: Pawel Jakub Dawidek <pjd_at_FreeBSD.org>
Date: Thu, 28 Jul 2005 10:28:44 +0200
On Thu, Jul 28, 2005 at 12:54:19AM -0700, Julian Elischer wrote:
+> it APPEARS that teh system is swapping out running programs in order to 
+> store more write data!
+> 
+> experiment:
+> boot to single user mode.
+> type:
+> mount {big partition}
+> dd if=/dev/zero bs=128K of=/$bigpartition}/bigfile count=1000000
+> 
+> notice that after a short while your dd is killed because the system is 
+> out of swapspace.
+> (it doesn't have any)
+> Why the F*ck does it need swapspace.? there are exactly 2 proceses 
+> running in userspace
+> and one of them s in wait4(). dd shows a resident size of about 170KB
+> leaving about a GIGABYTE of unused RAM.
+> 
+> The system should make dd wait rather than trying to swap its pages out..
+> 
+> 
+> if you then do
+> swapon (your swap device)
+> and repeat teh command in the background,
+> vmstat 1 will show you pages being faulted in and out...
+> no WONDER IO goes to hell in a handbasket..
+> 
+> Outgoing IO should never be able to force running programs out!
+> It should start re-using old pages from the same file!
+> 
+> 4.11 gives a consinstent 65MB/sec with this array, for as long as I run it..
+> 6.0 gives me 65MB for 15 seconds and then it drops to 20MB/sec and then 
+> 10MB/sec
+> and the swap disk bursts into life.
+> 
+> the array goes from all the lights solidly on, to bursts of activity 
+> with large gaps in between them.

It looks I observed the same behaviour!

I was testing GELI GEOM class and I was getting ENOMEM errors from malloc(9).
Then, I was sure I've a memory leak, but this was only 'vmstat -m' issue so
there was no memory leak and I shouldn't get ENOMEM in the first place while
copying /usr/src/sys directory.

I'm also able to reproduce your dd(1) test easly.

Not sure when it was introduced...

-- 
Pawel Jakub Dawidek                       http://www.wheel.pl
pjd_at_FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!

Received on Thu Jul 28 2005 - 06:28:46 UTC

This archive was generated by hypermail 2.4.0 : Wed May 19 2021 - 11:38:39 UTC