Finally, that's the first comment I get, instead of someone extolling the virtues of swap and babbling warmed-over 1990s rules of thumb about "twice the RAM is the recommended size of your swap file" as if I'm going to wait for even 200MB of swap to fill up before flipping out and killing the offending process, let alone 8GB.
>Finally, that's the first comment I get, instead of someone extolling the virtues of swap and babbling warmed-over 1990s rules of thumb about "twice the RAM is the recommended size of your swap file" as if I'm going to wait for even 200MB of swap to fill up before flipping out and killing the offending process, let alone 8GB.
Those people have doubtless misunderstood the point of swap. You should have a swapfile/partition because it allows allocated but currently unused memory (from an application which keeps data hanging around which is not needed for most of it's working life, or an application which simply leaks) to be dumped to long term storage, thus freeing memory for its real use: page cache. Sweet, sweet page cache.
I'm always happy to see a few tens, even a couple of hundred MB of swap in use, because it means that some application had some unused data hanging around for so long that to leave it there would mean my machine having to read from disk more frequently, which would be Bad.
Yes, and it may seem a merely semantic difference, but there's still a difference between "swap space" and "hibernation backing". One I'm willing to wait for while it fills, the other, I am not. The kernel may not distinguish, but I do. When I don't care about hibernation I just remove it.
Right, swapoff is totally acceptable imo if you're having trouble with swap. I was commenting on how "double the size of RAM" is a bad or outdated guideline. There's been a few times where I've regretted not making my swap space big enough, sometimes when getting a RAM upgrade and wanting to do hibernation, etc. My disk isn't super pressed for space so in my mind there's no real reason to be stingy, and then you don't have to grow/shrink partitions if an upgrade occurs after the initial disk setup.
Swap still provides one important function: it allows large, inactive, long-running processes to be moved from RAM to make room for more caches and buffers.
That said, if you never fill your RAM with buffers/cache, then of course swap makes no sense.
On the other hand, one could say that moving a large process from RAM to swap certainly makes it nearly inactive and long-running...
Memory is cheap. I'd rather pay a little more and have the long-running processes stay in memory than worry about my more active processes ending up in swap accidentally. If I wish to reserve 1GB for these long-running processes in RAM instead of swap, I can still win by buying 2GB more RAM.
Besides there's always some disk-backing you can't generally avoid: pages containing read-only executables can be purged from memory when unused and re-read from the original .so or binary when needed. This is something that would never go to swap anyway.
Stop measuring your swap space in terms of disk space, start imagining it in terms of "amount of time it takes to fill up". At a nice round 20MB/sec write, it would take a solid 400 seconds to use that much swap at full speed. In reality, you can't fill it that fast either, because it's seeking, also yanking other stuff out to run your other processes, and then sticking the stuff back in to run those other processes, so it's really a mess; the real amount of time it could take just to make initial real use of 8GB of swap could be days, no joke. (In other words, I don't care if you have a 80MB/s drive, it really doesn't matter much, plus swap usage tends to be full of seeks anyhow so it's not like you're going to get 20MB/s either.) Given that a modern system is, relatively speaking, brought to a near-complete halt by being in swap, what possible process are you going to run where you are willing to put up with your system being brought to a halt for even tens of minutes at a time, let alone the hours or days it'll take to fully utilize 8GB?
Clearly, you don't need and basically can't use an 8GB swap partition. So, how long are you willing to put up with? That will vary, but let's say 10 seconds before you're "flipping out and killing processes". That's a 200MB swap partition. But... that's only 5% of the size of your RAM! If that's the difference between a process completing or not, you've probably already lost. Or you should just kill Firefox.
On a 4GB system, the most likely reason a process is pushing you into swap is that it is in an infinite allocation loop, and all having 4GB of swap does is make your system crawl that much more before the process dies.
This is a result of RAM sizes increasing far faster than hard drive write speed has. When I had 32MB of RAM, it made sense to have some swap. I could swap out, say, 16MB of unused executable pages (bits of windows, bits of drivers I'm not using, bits of the massive Office suite I'm not using, etc) and get that much more working set, and this could happen in a reasonable time; the system choked for a couple of seconds but recovered in a stable manner. Now swapping out 16MB of executable is a joke. SSDs may change the balance, but these balances have been out of whack for a long time, I rather suspect that even with an SSD it won't be worth swap. Especially since by the time SSDs are truly common 8GBs of RAM may be entry-level because, well, why not? (Poking at Best Buy really quickly, at the $500 line you get 4GB for laptops, a little over $300 for desktops, coming down fast. I'm not sure they have anything less than 2GB now and even that is really into "don't use swap" for the average user.)
Please note that this doesn't apply all the same to servers. I have servers running with 16 GB of RAM and 16 GB of swap (because I simply can't even stop them to add RAM), and even then about twice a year they run out of memory and the OOM killer does its dirty job. However without swap they used to simply crash and burn, so swap is fine in this case.
The only thing I can possibly imagine that a server can do with 16GB of swap is leaking garbage like a sieve and then getting the garbage swapped out. On an incredibly local basis, yes, this might be a better idea than not having swap, but as a generalized reason for servers to have swap it's terrible. If that's not what's happening I am all ears as to what the situation really is, but I'm sure it'll be something very unusual; in general if your server has even the slightest need for performance it can't use swap.
Not a memory expert myself, but a friend of mine dismisses overly large swap as consuming more RAM to handle it's addressing - that is, the more swap you have, the less of your RAM you can access.
I'm not sure what the consumption rate is, but he won't create general servers with more than 2GB swap (if that), no matter how much RAM the system has.
You're probably never going to hit the swap for a good reason. You might hit it if you do a mistake (resizing an image by 5000% and not 500%), in which case you will suffer if you let your gigabytes of swap get filled), or if you have specific needs (video editing, and still...).
Keeping a swap partition in a 2011 computer is 1. a waste of disk space, 2. an unecessary matter of worries (2-1. may leak some infos, even if your other partitions are encrypted, if your computer is stolen, 2-2. a source of potential bugs).
And you're smart to do so. Swap is useless for 99% of end-user systems. ChromeOS doesn't use any swap partition, for instance.
As for hibernation, it's not available if you use a crypted swap, which is wise to use (at least on a laptop).
As a result, I, too, disabled swap completely.