macOS – How to Increase Maximum Allowed Swap Space

kernelmemory pressurevirtual-memory

When I try to do a calculation that needs to allocate and use 128 GB of memory (it is a command line program written in C), the kernel kills my process with extreme prejudice. This console log entry is an example of one instance:

9/25/15 7:08:40.000 PM kernel[0]: low swap: killing pid 6202 (huffgrp)

The calculation works fine and in a reasonable amount of time when it allocates and uses 64 GB of memory. My machine has 32 GB of RAM and beaucoup space on the hard drive. I also tried this on another machine with 8 GB of RAM, on which the 64 GB calculation runs fine as well, taking longer of course, but the 128 GB calculation gets killed by the kernel in the same way. On both I am running Yosemite 10.10.5.

By the way, malloc() never returns an error, no matter how much space I ask for. The kernel will only kill the process once too much of that memory is actually being used by the process, resulting in lots of swapping to the hard drive.

So there appears to be a secret swap space limit somewhere between 64 GB and 128 GB.

My question is: how do I reconfigure the kernel to permit more swap space? I found a promising looking file, /System/Library/LaunchDaemons/com.apple.dynamic_pager.plist, but I don't see the secret number in there. The man page for dynamic_pager says that all it does is set the name and location of the swap files. There is an older version of the same man page that documents a -S option to set the size of the swapfiles created. I tried that, requesting 160 GB swapfiles, but it had no effect. The swapfiles were still 1 GB each, and the process was still killed by the kernel.

Best Answer

While I could not find any more recent documentation, Singh's "Mac OS X Internals: A Systems Approach" mentions two obscure limits in /sbin/dynamic_pager (p. 919 f.):

The absolute minimum and maximum swap file sizes are 64MB and 1GB, respectively.

The maximum swap file size must not be greater than 12.5% of the free space available on the volume that contains the swap file. Moreover, the maximum swap file size must not be greater than the amount of physical memory on the system.

At most, eight swap files can be created.

[...]

While the book is a bit dated and the numbers are likely to have changed, the 12.5% rule sounds very suspicious to me.

Singh implies that those are hard-coded limits in the binary and thus the property file you found is likely not to be of use. Also note that the swap system creates several, but only up to eight, distinct swap files.

/sbin/dynamic_pager is documented, as you noted, as a very rudimentary man page, and there is a conflicting statement in the current developer documentation on Virtual Memory, stating that

To give processes access to their entire 4 gigabyte or 18 exabyte address space, OS X uses the hard disk to hold data that is not currently in use. As memory gets full, sections of memory that are not being used are written to disk to make room for data that is needed now. The portion of the disk that stores the unused data is known as the backing store because it provides the backup storage for main memory.

[...]

Note: Unlike most UNIX-based operating systems, OS X does not use a preallocated disk partition for the backing store. Instead, it uses all of the available space on the machine’s boot partition.

(emphasis mine)

Hence, at around this point, I'd go for some pragmatic workaround. Perhaps try increasing your disk space even further to check for a correlation in the obtainable swap size, thus checking the 12.5% limitation mentioned above.

Related Solutions

Virtual Memory – How to Move Swap Files to Different Drive on MacOS

note: copied from SuperUser:

For older versions of OSX, see this thread on forums.macosxhints.com.

Credit goes to dblu for explaining the use of plutil, ZILjr for introducing the wait4path command, and ekl for simplifying the whole thing by eliminating the need for an intermediate shell script.

Complete solution:

1. Open Terminal and make a backup copy of Apple's default dynamic_pager.plist:

$ cd /System/Library/LaunchDaemons
$ sudo cp com.apple.dynamic_pager.plist{,_bak}

2. Convert the plist from binary to plain XML:

$ sudo plutil -convert xml1 com.apple.dynamic_pager.plist

3. Open the converted plist with your text editor of choice. (I use pico, see dblu's answer for an example using vim):

$ sudo pico -w com.apple.dynamic_pager.plist

It should look as follows:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs$
<plist version="1.0">
<dict>
    <key>EnableTransactions</key>
    <true/>
    <key>HopefullyExitsLast</key>
    <true/>
    <key>Label</key>
    <string>com.apple.dynamic_pager</string>
    <key>OnDemand</key>
    <false/>
    <key>ProgramArguments</key>
    <array>
        <string>/sbin/dynamic_pager</string>
        <string>-F</string>
        <string>/private/var/vm/swapfile</string>
    </array>
</dict>
</plist>

4. Modify the ProgramArguments array (lines 13 through 18) to use the wait4path shell command prior to launching dynamic_pager. See note #1 for details on why this is necessary. In the following example, my partition is called Swap, and I chose to put the swapfiles in a hidden directory on that partition, called .vm be sure that the directory you specify actually exists. The XML should look as follows:

<key>ProgramArguments</key>
<array>
    <string>/bin/bash</string>
    <string>-c</string>
    <string>/bin/wait4path /Volumes/Swap/ &amp;&amp;
/sbin/dynamic_pager -F /Volumes/Swap/.vm/swapfile</string>
</array>

5. Save the plist, and return to the terminal prompt. Using pico, the commands would be:

<ctrl+o> to save the file
<enter>  to accept the same filename (com.apple.dynamic_pager.plist)
<ctrl+x> to exit

6. Convert the modified plist back to binary:

$ sudo plutil -convert binary1 com.apple.dynamic_pager.plist

7. Restart your Mac. If you run into trouble, switch to verbose startup mode by holding down Command-v immediately after the startup chime. This will let you see all of the startup messages that appear during startup. If you run into even worse trouble (i.e. you never see the login screen), hold down Command-s instead. This will boot the computer in single-user mode (no graphical UI, just a command prompt) and allow you to restore the backup copy of com.apple.dynamic_pager.plist that you made in step 1.

8. Once the computer boots, fire up Terminal and verify that the swap files have actually been moved:

$ cd /Volumes/Swap/.vm
$ ls -l

You should see something like this:

-rw-------  1 someUser  staff  67108864 18 Sep 12:02 swapfile0

9. Delete the old swapfiles:

$ cd /private/var/vm
$ sudo rm swapfile*

10. Profit!

Note 1

Modifying the arguments to dynamic_pager in the plist without using wait4path does not always work, and when it fails, it does so in a spectacularly silent way. The problem stems from the fact that dynamic_pager is launched very early in the startup process. If your swap partition has not yet been mounted when dynamic_pager is first loaded (in my experience, this happens 99% of the time), then the system will fake its way through. It will create a symbolic link in your /Volumes directory which has the same name as your swap partition, but points back to the default swapfile location (/private/var/vm). Then, when your actual swap partition mounts, it will be given the name Swap 1 (or YourDriveName 1). You can see the problem by opening up Terminal and listing the contents of your /Volumes directory:

$ cd /Volumes
$ ls -l

You will see something like this:

drwxrwxrwx  11 yourUser  staff   442 16 Sep 12:13 Swap -> private/var/vm
drwxrwxrwx  14 yourUser  staff     5 16 Sep 12:13 Swap 1 
lrwxr-xr-x   1 root      admin     1 17 Sep 12:01 System -> /

Note that this failure can be very hard to spot. If you were to check for the swapfiles as I show in step 12, you would still see them! The symbolic link would make it seem as though your swapfiles had been moved, even though they were actually being stored in the default location.

Note 2

I was originally unable to get this to work in Snow Leopard because com.apple.dynamic_pager.plist was stored in binary format. I made a copy of the original file and opened it with Apple's Property List Editor (available with Xcode) in order to make changes, but this process added some extended attributes to the plist file which caused the system to ignore it and just use the defaults. As dblu pointed out, using plutil to convert the file to plain XML works like a charm.

Note 3

You can check the Console application to see any messages that dynamic_pager_init echos to the screen. If you see the following lines repeated over and over again, there is a problem with the setup. I ran into these messages because I forgot to create the '.vm' directory that I specified in dynamic_pager_init.

com.apple.launchd[1]  (com.apple.dynamic_pager[176]) Exited with exit code: 1
com.apple.launchd[1]  (com.apple.dynamic_pager) Throttling respawn: Will start in 10 seconds

When everything is working properly, you may see the above message a couple of times only, and then no more of the "Throttling respawn" messages. This means that the system did have to wait for the partition to load, but in the end it was successful.

Exclude the path from Time Machine

In OS X 10.7 and later you can do this with tmutil. Example:

sudo tmutil addexclusion -p /Volumes/Swap/.vm

For that example, to review the result:

tmutil isexcluded /Volumes/Swap && tmutil isexcluded /Volumes/Swap/.vm

If the volume used for swap need not be indexed

Use mdutil. Example, to switch off then erase the store:

sudo mdutil -i off /Volumes/Swap && sudo mdutil -E /Volumes/Swap

In Activity Monitor, how can Swap Used be more than Page Outs

My understanding is this:

Program needs access to some data. Attempts to do so, and fails, due to lack of RAM.
OS takes control. There is a page-out operation, where physical RAM is copied to swap.
There is a page-in operation, where the required data is copied to RAM.
OS gives control back to program.
Program tries again, finds its data.

OS's are getting smart about how they choose what and when to to page in/out. So the operation counts may not be the same, and the swap space may perhaps be in use without there being page outs.

Unless of course I'm backwards on what a page-in/page-out operation is. In which case your question makes perfect sense.