Origin of the rule that swap size should be 2x of the physical memory
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Origin of the rule that swap size should be 2x of the physical memory
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Re: Origin of the rule that swap size should be 2x of the physical memory
#2Curious, How much swap have you personally allocated on your personal setup?
Re: Origin of the rule that swap size should be 2x of the physical memory
#3The OP clearly states that he wants to know the earliest origin of the rule, and the only answers he gets are people giving their own opinions on how much swap space you should have.
Too bad because it's an interesting question that I would also like to know the answer to.
Re: Origin of the rule that swap size should be 2x of the physical memory
#4Curious, How much swap have you personally allocated on your personal setup?
Why was this downvoted? I’m generally curious what current recommendations for swap are too!
Edit: oh and I don’t have an actual personal system with swap configuration on it anymore to give my own answer anymore either.
Re: Origin of the rule that swap size should be 2x of the physical memory
#5Curious, How much swap have you personally allocated on your personal setup?
Zero. My office workstation has 48 GB of RAM, my home computer has 64 (I went a bit overboard). I have very bad memories of swap thrashing and the computer becoming totally unresponsive until I forced a reset; if I manage to fill up so much RAM, I very much prefer the offending process to die instead of killing the whole computer.
Re: Origin of the rule that swap size should be 2x of the physical memory
#6It's old enough that I'd put money on DEC. Any takers on that.
Re: Origin of the rule that swap size should be 2x of the physical memory
#7Curious, How much swap have you personally allocated on your personal setup?
My work laptop currently has 96GB of RAM. 32 of it is allocated to the graphics portion of the APU. I have 128GB (2x) of SWAP allocated, since I sometimes do big FPGA Synthesizations, which take up 50GB of RAM on its own. Add another two IDEs and a browser, and my 64GB or remaining RAM is full.
Re: Origin of the rule that swap size should be 2x of the physical memory
#8Curious, How much swap have you personally allocated on your personal setup?
64GB RAM, zero swap. Until recently RAM was cheap so swap made little sense when you could simply buy more RAM.
Re: Origin of the rule that swap size should be 2x of the physical memory
#9The OP clearly states that he wants to know the earliest origin of the rule , and the only answers he gets are people giving their own opinions on how much swap space you should have. Too bad because it's an interesting question that I would also like to know the answer to.
Nope. Those are not the only answers I am seeing. I’m still curious though. 2x was nice because nobody really questioned it. Now that we have there doesn’t seem to be one “answer”. This is a fun/interesting question that comes up every now and then here and elsewhere :-)
I suspect someone smarter than me about system tuning will have a much smarter and nuanced answer than “just use 2x”
Re: Origin of the rule that swap size should be 2x of the physical memory
#10Early BSD VM pre-allocated swap backing for every anonymous page — you couldn't allocate virtual memory without a swap slot reserved for it, even if the page was never paged out.
When a process forks, the child needed swap reservations for the parent's entire address space (before exec replaces it). A large process forking temporarily needs double its swap allocation. If your working set is roughly equal to physical RAM, fork alone gets you to 2x.
This was the practical bottleneck people actually hit. Your system had enough RAM, swap wasn't full, but fork() failed because there wasn't enough contiguous swap to reserve. 2x was the number that made fork() stop failing on a reasonably loaded system.
The later overcommit/copy-on-write changes made this less relevant, but the rule of thumb outlived the technical reason. Most people repeating "2x RAM" today are running systems where anonymous pages aren't swap-backed until actually paged out.
Today swap is no longer about extending your address space, it's about giving the kernel room to page out cold anonymous pages so that RAM can be used for disk cache.
A little swap makes the system faster even when you're nowhere near running out of memory, because the kernel can evict pages it hasn't touched in hours and use that RAM for hot file data instead.
The exception is hibernation — you need swap >= RAM for that, which is why Ubuntu's recommendations are higher than RedHat's 20% of RAM.