Subject: Re: Bad response...
To: Jochen Kunz <>
From: Steven M. Bellovin <>
List: current-users
Date: 08/30/2004 09:43:32
In message <>, Jochen Kunz =
>On Fri, Aug 27, 2004 at 11:20:14AM +0200, Johnny Billquist wrote:
>> I'm getting tired of the fact that my system's response have gone down=

>> dramatically the last year or so.
>> I believe the problem is in the unified memory buffer handling. The di=
sk =

>> cache is way to agressive in grabbing memory.
>Seconded. I have seen the same on other OS (e.g. Tru64) too. The
>filesystem buffer cache eats up all memory, executable pages are reused
>resulting in heavy paging when an application process is touched again. =

>This is really annoying.

Agreed -- I haven't been really satisfied with interactive performance =

since UBC came in.  Now that my major machines have 512M apiece, I can =

live with it, but it's still not what I'd like.

I think there are at least two problems.  First, using arbitrary =

amounts of memory for file cache leaves lots of dirty pages; these need =

to be cleaned before the memory can be reused.  By contrast, pages =

used for process text can be reused immediately.  =

But I suspect that a bigger problem is that once there's a demand for =

memory to satisfy an interactive process, whether a new command or some =

process that has just awakened, it looks like the new process has to =

fight with the file-writing processes for the newly-freed memory.  I =

suspect that the percentage of RAM allocated to file writes needs to =

follow a strategy similar to TCP's congestion control -- when there are =

competing requests, it needs to *seriously* cut back the percentage =

allocatable to such needs -- say, an additive increase/multiplicative =

decrease scheme, just like TCP uses.

		--Steve Bellovin,