Comments on: Who can actually use this?

By: Sam

Sam — Fri, 15 Aug 2008 23:26:30 +0000

When I first looked at the patch it seemed like just a clean little change which added the benefit of a higher addressable swap space.

However I was wondering, if you were clustering systems which can migrate processes between machines, a shared swap space might seem a prudent step, processes could have been paged out on one machine only to be paged in on another following migration. I am not sure you would migrate a process that had not been swapped out as you would want the advantage of locality, but if it had it seems like a likely candidate for migration.

The amount of space required could be maximum amount of combined RAM times some multiple, but if there is no criteria as to how many machines might be a part of the cluster from one moment to the next, you would probably be going for as much space as can be spared for swapping.

It would not work quite like normal swap and there are plenty of cases I haven’t considered, and you can probably tell the mechanism of migration is still an unknown to me, but the basic idea *seems* right. I don’t know, it was just a thought, I might be quite wrong.

By: adamo

adamo — Tue, 12 Aug 2008 10:10:38 +0000

>>But in a very few years time it will be a reality.

>Why? Because you arenâ€™t able to administer your machine correctly?

Because I may need more RAM than the machine can provide. Rare exceptions do happen.

By: corecode

corecode — Mon, 11 Aug 2008 20:45:24 +0000

right now we can only use it by having a lot of processes which all consume a lot of memory. Wait for amd64 and we can use this all by one process :)

By: BSD News » Blog Archive » Terabyte of Swap space.

BSD News » Blog Archive » Terabyte of Swap space. — Mon, 11 Aug 2008 14:14:55 +0000

[…] recent commit from Matthew Dillon enables use of at least a terabyte of swap space. Â Is there anyone who can actually use that much yet? Â Swap is traditionally 2x available memory, […]

By: Justin Sherrill

Justin Sherrill — Mon, 11 Aug 2008 13:45:57 +0000

I had said “traditionally”, because I know it’s no longer as accurate a rule as it used to be, but I haven’t yet heard a rule to replace it.

Searching around the web in places like the FreeBSD Handbook or HP-UX documentation shows calls for 2 to 4 times as much swap as available memory, so perhaps that rule of thumb still holds.

By: daemonic

daemonic — Mon, 11 Aug 2008 09:38:27 +0000

>Swap is traditionally 2x available memory

It’s a tradition and it’s mere nonsense.

>But in a very few years time it will be a reality.

Why? Because you aren’t able to administer your machine correctly?

By: adamo

adamo — Mon, 11 Aug 2008 09:21:26 +0000

OK maybe today it may seem as overkill. But in a very few years time it will be a reality. However, in today’s use, imagine that you have a data structure (like a B-tree variant) and you have implemented it as a main memory only data structure. It is not impossible to have a need to index data of this size (and even compromise using swap space).

By: Joe "Floid" Kanowitz

Joe "Floid" Kanowitz — Mon, 11 Aug 2008 04:44:30 +0000

FB-DIMMs are up to 16GB now. It’s not unimaginable!