Reinier Zwitserloot wrote: > Kirk: Last time I owned a windows box (I admit, that was quite a while > ago, but it did have XP SP2, which as far as everyone is concerned is > the 'latest' version of windows still, I believe), when I tried to > turn swap off a whole host of apps stopped working. I then configured > swap to be 50MB in size, and they started working again, but I didn't > notice any significant speedup; instead, some research showed that the > 50MB chunk was heavily contested. I then loaded up the old ramdisk > driver to create an all-RAM disk just to satisfy XP's insistence on > swap space, but that dropped the entire kernel down to 16-bit mode or > some such whacky legacy support maneuvre. A year later I downloaded an > XP native driver to create ram disks but never got around to trying > this again. > humm, interesting. I turned swap down to zero. The system complained but then I got about a 10% increase in performance and no startup pause while the system swapped in applications that had been swapped out.
The problem with swap is that it is an easy target for the systems guys to stuff functionality into. I learned this the hard way while working with Unicos (Cray unix). Cray's have no virtual memory so if you need to re-organize an application say after a malloc, the system would have to do what is known as a roll out, repack, and roll in. That is to say, the memory for the application would be written out to disk, and reorganized on the way back into memory. As you could imagine, it isn't something that you wanted to have happen. As you can see, being able to do that with swap is significantly easier. In Windows, application working set maintenance depends on swap. If you have applications that require a lot of maintenance, which it appears that you have, then I'd expect that turning off swap would unfortunately, not be a great idea. > I'm on OS X now, but I doubt turning off paging is a good idea; even > if I have more than enough RAM to keep all running apps in memory, > swapping unused bits out is still a useful exercise as it frees up RAM > for disk caches. I wonder if the extra effort to write RAM to swap is > more, or less, disk access compared to the # of accesses saved by disk > caches. > Again, Unix swap is heavily overloaded. I'm also using OSX and I've not even tried to turn it off. My guess is that even sleep on Mac is dependent upon it. A shame really 'cos I can see how performance suffers because of it. Solaris page alloc depends upon swap and so on... It is really time to start changing this as we've got plenty of RAM and the gap between disk access and ram access is enormous. I guess SSD is coming to save the day. I've been waiting a couple of years now for capacity to increase and price to decrease on SSD drives. I give it another year for capacity and 2 for price. Regards, Kirk > On Jan 1, 10:11 pm, kirk <[email protected]> wrote: > >>> A recap of issues with eating as much memory as you want: >>> >>> FACT A: OSes manage virtual memory and make it relatively hard for >>> userland apps to meddle and/or inspect this. >>> >>> FACT B: There's no way for the JVM to see any difference between an >>> app leaking memory and an app that has a legitimate use for its >>> continued increased memory requirements. Even if we can somehow >>> magically create a garbage collector that can do perfect cleanup in >>> near zero extra time compared to the more heuristic (pretty good but >>> not perfect) approach used by the current garbage collector, that >>> still doesn't allow us to assume that a JVM that wants more memory >>> should really get it. >>> >> Actually... I've written about how one should be able to automagicly >> detect memory leaks. Leaked objects are never accessed so if you track >> access, you can find them. I had some discussions with the guy working >> on the G1 collector and while there are some performance issues with >> tracking references, the scheme was workable. While sorting through >> details, a couple of papers came to light that detail similar schemes >> for automatic leak detection. If I can work out the issue with write >> barriers, this is something that could be dropped into OpenJDK.> FACT C: Its >> possible to do a complete garbage collect, but this takes >> >>> precious CPU cycles. This is worth it to avoid swapping, but it is a >>> complete waste of time if the host computer has real memory to spare. >>> It's not like unused memory saves power or some such. It's effectively >>> a free resource, if its there. >>> >> swap is an archaic optimization where disk is traded to create more >> memory. Out machines now have more memory than one can shake a stick at. >> If you've got 2-4 gigs on your machine, I'd turn swap off. I've done >> this for my windows machines for a couple of years now. They have 2 gigs >> of ram. Occasionally I have to turn swap back on when I'm doing >> something that requires a lot of ram. However I normally use less than >> 2gigs so turning off swap is no big deal. The performance boost it gives >> is worth it. It also keeps my disk drive quiet which is a blessing for >> the batteries in my laptops. >> >> Regards, >> Kirk >> > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "The Java Posse" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/javaposse?hl=en -~----------~----~----~----~------~----~------~--~---
