An unbelievable amount of junk is being posted. The short answer is: Always run with swap installed on a Linux or BSD system, period. If you don't, you're an idiot. As to why? There are many, many reasons. How much? Generally as much as main memory. If you have a tiny amount of memory, more, if you have tons of memory, like a terrabyte, then less. And these days it absolutely should be on a SSD. SSDs are not expensive... think about it. Do you want 40GB of swap? It's a $20 SSD. Frankly, just putting the swap on your main SSD (if you have one) works just as well. It won't wear out from paging.
Linux and BSD kernels are really good at paging out only what they really need to page out, only paging when there is actual memory pressure, doing it in batches (no need to worry about SSD swap write amplification much these days... writes to swap are batched and are actually not so random. Its the reads that tend to be more random). I started using SSDs all the way back in the days of the Intel 40GB consumer drives, much of them for swap, and have yet to reach the wear limit for any of them. And our machines get used heavily. SSDs wear out from doing other stupid things... swap is not usually on the list of stupid things. The days of just random paging to swap for no reason are long over. Windows... YMMV.
Without swap configured you are wasting an enormous amount of relatively expensive ram to hold dirty data that the kernel can't dispose of. People really underestimate just how much of this sort of data systems have... it can be huge, particularly now with bloated browsers like Chrome, but also simply things like TMPFS which is being used more heavily every day. Without swap configured if memory gets tight the ONLY pages the kernel can evict are shared read-only file-backed pages.... generally 'text' pages (aka code). These sorts of pages are not as conducive to paging as data pages and it won't take long for the system to start to thrash (this is WITHOUT swap) by having to rip away all the program code and then instantly page it back in again. WITH swap, dirty data pages can be cleaned by flushing them to swap.
Configure swap, use SSDs. If you are worried about wear, just check the wear every few months but honestly I have never worn out a SSD by having swap configured on it.... and our systems can sometimes page quite heavily when doing bulk package builds. Sometimes as much as 100GB might be paged out, but it allows us to run much more aggressive concurrency settings and still utilize all available CPU for most of the bulk run.
So here are some bullets.
1. Systems treat memory as SWAP+RAM, unless you disable over-commit. Never disable over-commit on a normal system. The SWAP is treated like a late-level cache. CPU, L1, L2, L3, [L4], RAM, SWAP. Like that. The kernel breaks the RAM down into several queues... ACTIVE, INACTIVE, CACHE, then SWAP. Unless the system is completely overburdened, a Linux or BSD kernel will do a pretty damn good job keep your GUI smooth even while paging dead browser data away.
2. Kernels do not page stuff out gratuitously. If there is no memory pressure, there will be no paging, even if the memory caches are not 'balanced'.
3. There is absolute no reason to waste memory holding dirty data from idle programs or browser tabs. If you are running a desktop browser, swap is mandatory and your life will be much better for it.
4. Same is true for (most) SERVERs. Persistent sessions are the norm these days, and 99% of those will be idle long-term. With swap the server can focus on the ones that aren't, and paging in an idle session from a SSD takes maybe 1/10 of a second.
5. CPU overhead for paging is actually quite low, and getting lower every day. Obviously if a program stalls on a swapped page that has to be paged in you might notice it, but the actual CPU overhead is almost zip.
6. The RAM required to manage swap space is approximately 1 MByte per 1 GByte of swap. I regularly run hundreds of gigabytes of swap, just to give me breathing room if I happen to get runaways. System ram overhead is just not a big deal.
7. SSD wear is not typically an issue these days, for many reasons. Writes to swap are typically batched and actually don't have all that much write amplification. Its the reads which tend to be more random and random reads are the perfect load for a SSD.
Run with a SSD, configure a reasonable amount of swap on it, stop worrying about paging-induced wear, and you are done. This is the modern world.
-Matt
Linux and BSD kernels are really good at paging out only what they really need to page out, only paging when there is actual memory pressure, doing it in batches (no need to worry about SSD swap write amplification much these days... writes to swap are batched and are actually not so random. Its the reads that tend to be more random). I started using SSDs all the way back in the days of the Intel 40GB consumer drives, much of them for swap, and have yet to reach the wear limit for any of them. And our machines get used heavily. SSDs wear out from doing other stupid things... swap is not usually on the list of stupid things. The days of just random paging to swap for no reason are long over. Windows... YMMV.
Without swap configured you are wasting an enormous amount of relatively expensive ram to hold dirty data that the kernel can't dispose of. People really underestimate just how much of this sort of data systems have... it can be huge, particularly now with bloated browsers like Chrome, but also simply things like TMPFS which is being used more heavily every day. Without swap configured if memory gets tight the ONLY pages the kernel can evict are shared read-only file-backed pages.... generally 'text' pages (aka code). These sorts of pages are not as conducive to paging as data pages and it won't take long for the system to start to thrash (this is WITHOUT swap) by having to rip away all the program code and then instantly page it back in again. WITH swap, dirty data pages can be cleaned by flushing them to swap.
Configure swap, use SSDs. If you are worried about wear, just check the wear every few months but honestly I have never worn out a SSD by having swap configured on it.... and our systems can sometimes page quite heavily when doing bulk package builds. Sometimes as much as 100GB might be paged out, but it allows us to run much more aggressive concurrency settings and still utilize all available CPU for most of the bulk run.
So here are some bullets.
1. Systems treat memory as SWAP+RAM, unless you disable over-commit. Never disable over-commit on a normal system. The SWAP is treated like a late-level cache. CPU, L1, L2, L3, [L4], RAM, SWAP. Like that. The kernel breaks the RAM down into several queues... ACTIVE, INACTIVE, CACHE, then SWAP. Unless the system is completely overburdened, a Linux or BSD kernel will do a pretty damn good job keep your GUI smooth even while paging dead browser data away.
2. Kernels do not page stuff out gratuitously. If there is no memory pressure, there will be no paging, even if the memory caches are not 'balanced'.
3. There is absolute no reason to waste memory holding dirty data from idle programs or browser tabs. If you are running a desktop browser, swap is mandatory and your life will be much better for it.
4. Same is true for (most) SERVERs. Persistent sessions are the norm these days, and 99% of those will be idle long-term. With swap the server can focus on the ones that aren't, and paging in an idle session from a SSD takes maybe 1/10 of a second.
5. CPU overhead for paging is actually quite low, and getting lower every day. Obviously if a program stalls on a swapped page that has to be paged in you might notice it, but the actual CPU overhead is almost zip.
6. The RAM required to manage swap space is approximately 1 MByte per 1 GByte of swap. I regularly run hundreds of gigabytes of swap, just to give me breathing room if I happen to get runaways. System ram overhead is just not a big deal.
7. SSD wear is not typically an issue these days, for many reasons. Writes to swap are typically batched and actually don't have all that much write amplification. Its the reads which tend to be more random and random reads are the perfect load for a SSD.
Run with a SSD, configure a reasonable amount of swap on it, stop worrying about paging-induced wear, and you are done. This is the modern world.
-Matt
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