Messages in this thread | | | Date | Sat, 03 Mar 2007 16:30:56 -0500 | From | Rik van Riel <> | Subject | Re: userspace pagecache management tool |
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Andrew Morton wrote: > On Sat, 03 Mar 2007 15:40:42 -0500 Rik van Riel <riel@redhat.com> wrote:
>> I am sick and tired of the "this is hard, let userspace do it" attitude. > > Anything you try to do in-kernel will catastrophically screw up some > workloads. You don't have a chance of getting this right.
Any time you follow the directions of one userspace program, you can screw up others. I suspect that userspace has far less of a chance of getting it right than the kernel.
ALSA would be a good example of why it is bad to export tuning knobs directly to userspace - many sound cards have non-standard names for the volume controls, making it almost impossible for userspace to present the user with a simple user interface for tweaking the volume.
> You are the kernel. The user just read an entire kernel tree. You face a > binary decision: do you cache that tree or do you not? Your time starts > now. What is your answer?
Lets turn this around.
The user has been accessing the kernel tree over and over again, for hours on end (compile testing a patch). Along comes a backup program, that tells you to evict the whole thing from the cache.
What do you do?
How can you make global policy decisions based on the intent of one program?
Only the kernel knows the state of the whole system and has observed the behaviour of all the processes. One process has no idea what the other processes in the system are doing.
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