Rather than re-write your scripts to store temp files into /dev/shm, you can just mount /tmp using the tmpfs file system and get the same benefit for all your programs. Some distros do this by default.
The relevant line from fstab is:
tmpfs /tmp tmpfs noatime 0 2
Now any program that writes to /tmp will be writing to a RAM disk, thus sparing unnecessary wear on my SSD.
I do mention this offhand in the article: "The existence of /dev/shm is a boon for me mostly because it means I never have to worry about whether /tmp is really RAM-based again."
Now you have to worry about whether you can access /dev/shm. Please encourage people to use supported interfaces instead of random voodoo (anything under /dev that wasn't there in 1995) for day-to-day tasks.
$ ls -ld /dev/shm
drwxrwxrwt 3 root root 120 Jun 32 02:47 /dev/shm/
Incidentally, "30 years ago" is the cutoff date for music being considered the oldies. This just made me realize Nevermind is now an oldie, and soon The Lonesome Crowded West will be too.
A past role in a past life had me installing security services on servers. One server had incredibly awkward permission sets across its common directories so our deployment script failed. The fix? Just throw it into /dev/shm and install it directly from there. It worked great.
> /dev/shm is typically world-writable by default:
You are relying on random implementation details instead of universal APIs that work across OSes and environments. Please stop.
So help me God, if I make a Linux system, I will make it _not_ have a /dev/shm just to avoid people relying on non-standard stuff for no good reason. Honestly, it's because of stuff like this that we need Docker.
I'm not really seeing a right or wrong here anyway unless you're distributing a script that's meant to run on all sorts of Linux systems. In which case you probably aren't concerned with the physical storage medium being used.
It's because of people doing random nonstandard shit that we need to Docker-ize a lot of software these days. People refuse to lift a single finger to adhere to conventions that let programs co-exist without simulating a whole god damn computational universe for each damn program.
/tmp is not specified to be a RAM disk by POSIX. Just that things in there are considered to be not persistent after a program stops (with implications for backups and disaster recovery). Sure, RAM disks work if the amount of /tmp space you need is less than your free physical RAM but sometimes that's not the case, either.
Back in the day you might place /tmp in a good spot for random access of small files on a disk platter. /var is vaguely similar but intended for things that need to be persistent.
Anyway it's not uncommon for systems to persist /tmp and clean it periodically from cron using various retention heuristics.
Ultimately POSIX concepts of mountpoints are strongly tied to optimizing spinning rust performance and maintenance and not necessarily relevant for SSD/NVME.
> /tmp
A directory made available for applications that need a place to create temporary files. Applications shall be allowed to create files in this directory, but shall not assume that such files are preserved between invocations of the application.
It doesn't say anything about what it's backed by.
file-hierarchy(7) states /dev/shm is tmpfs and that "all users have write access to this directory", so I think you'd have to be making a non-systemd distro
> Usually, it is a better idea to use memory mapped files in /run/ (for system programs) or $XDG_RUNTIME_DIR (for user programs) instead of POSIX shared memory segments, since these directories are not world-writable and hence not vulnerable to security-sensitive name clashes.
$XDG_RUNTIME_DIR usually points to /run/user/${uid}, so you're guaranteed that other users won't write there, and possibly won't even be able to read there.
I did this for a while, but writing files to ram can be dangerous, since most things assume unlimited disk space. I noticed that updates would fail on machines that had 16 gigs of ram unless I logged out of my window manager and did it from the TTY. Took quite a long time to realize it was because of all the compiles writing to /tmp. Much easier to just let the SSD get used.
You know what, your comment actually reminds me that this happened when I also had a bug in my configuration that was causing me to not actually use swap. I assume running out of tmpfs uses swap like anything else? I might give tmpfs another try.
This is an unnecessary optimization, particularly for the article's use case (small files that are read immediately after being written). Just use /tmp. The linux buffer cache is more than performant enough for casual usage and, indeed, most heavy usage too. It's far too easy to clog up memory with forgotten files by defaulting to /dev/shm, for instance, and you potentially also take memory away from the rest of the system until the next reboot.
For the author's purposes, any benefit is just placebo.
There absolutely are times where /dev/shm is what you want, but it requires understanding nuances and tradeoffs (e.g. you are already thinking a lot about the memory management going on, including potentially swap).
It's true that with small files, my primary interest is simply not to wear on my disk unnecessarily. However I do also often do work on large files, usually local data processing work.
"This optimization [of putting files directly into RAM instead of trusting the buffers] is unnecessary" was an interesting claim, so I decided to put it to the test with `time`.
$ # Drop any disk caches first.
$ sudo sh -c 'sync; echo 3 > /proc/sys/vm/drop_caches'
$
$ # Read a 3.5 GB JSON Lines file from disk.
$ time wc -l /home/andrew/Downloads/kaikki.org-dictionary-Finnish.jsonl
255111 /home/andrew/Downloads/kaikki.org-dictionary-Finnish.jsonl
real 0m2.249s
user 0m0.048s
sys 0m0.809s
$ # Now with caching.
$ time wc -l /dev/shm/kaikki.org-dictionary-Finnish.jsonl
255111 /dev/shm/kaikki.org-dictionary-Finnish.jsonl
real 0m0.528s
user 0m0.028s
sys 0m0.500s
$
$ # Drop caches again, just to be certain.
$ sudo sh -c 'sync; echo 3 > /proc/sys/vm/drop_caches'
$
$ # Read that same 3.5 GB LSON Lines file from /dev/shm.
$ time wc -l /dev/shm/kaikki.org-dictionary-Finnish.jsonl
255111 /dev/shm/kaikki.org-dictionary-Finnish.jsonl
real 0m0.453s
user 0m0.049s
sys 0m0.404s
Compared to the first read there is indeed a large speedup, from 2.2s down to under 0.5s. After the file had been loaded into cache from disk by the first `wc --lines`, however, the difference dropped to /dev/shm being about ~20% faster. Still significant, but not game-changingly so.
I'll probably come back to this and run more tests with some of the more complex `jq` query stuff I have to see if we stay at that 20% mark, or if it gets faster or slower.
A couple things to consider when benchmarking RAM file I/O verses disk-based file system I/O.
1 - Programs such as wc (or jq) do sequential reads, which benefit from file systems optimistically prefetching contents in order to reduce read delays.
2 - Check to see if file access time tracking is enabled for the disk-based file system (see mount(8)). This may explain some of the 20% difference.
Hard disagree. Disk buffer cache is too eager on writes (which makes sense for the usual case), so temporary data written to a filesystem are almost always written to the medium. With several GBs of temporary data it easily could fill up internal SSD write buffers and make whole system choppy.
My use case is to use yt-dlp to download videos to ramfs, watch them and then delete. Before i switched to ramfs, the final pass of yt-dlp (where audio and video tracks are merged to one file) ordinarily caused the issue with choppy system.
This only stands for modern storage devices with a controller. For SD cards that don't have wear leveling, writing to the same region will make it die faster.
> It's far too easy to clog up memory with forgotten files by defaulting to /dev/shm, for instance, and you potentially also take memory away from the rest of the system until the next reboot.
Aren't both solved by swapping?
Although I suppose on Linux, neither having swap, nor it being backed by dynamically growing files, is guaranteed.
For decades now I have been using RAM as "disk", i.e., boot from USB, no internal disk, no internal SSD.^1 Entire OS fits in memory. When compiling OS userland there is simply no comparison; disk is painfully slow. I am not a gamer. I use RAM as a filesystem and workspace. I just got another computer with 64GB RAM. I will get 96GB or more when price comes down.
1. I use removable, external drives for anything I want to save long-term. No "cloud" storage.
For those interested in a windows equivalent, with bells and whistles, ImDisk [0] has a nice ramdisk creation utility with options such as allocating memory dynamically, ntfs compression, and more.
For the more venturous there is GPURamDrive [1] , not as many options, as it was made as a more of an experiment, but with gpu's adding more and more vram, why not?
Speaking of RAM and disks, does anyone know what happens if you structure LVM volume such that there is a RAM based tmpfs as a front cache? Consistency issues aside, could it increase performance? Suppose I have an application that behaves such that it has very IO heavy write buffer of around 100GB with 10x or so NVMe backed storage for more rarely used data. Would
you do something else? The main problem I have currently is that the NVMes overheat occasionally from high IOPS which adds a lot of latency variance.
An assumption I’ve been revisiting is if I really do need to be writing to disk all the time. I can’t remember the last time I actually had a crash or other event where I would have abruptly lost my work.
I’m wondering if I can completely hide away the detail where I can work exclusively in memory (even when I habitually save my code) and “reconcile” as some task I do before shutdown.
In fact, that doesn’t even feel necessary… I git push my day’s work a number of times. None of that needs a local disk. And 64GB of memory was surprisingly affordable.
You might be interested in Tiny Core Linux [0], then, especially piCore. After the initial read from the persistent media, everything is in RAM, the entire filesystem. You are working exclusively in memory until and unless you run a specific command to save everything you care to save back to that media again.
I have it running on a Raspberry Pi so that my already sparingly-used SD card's lifespan gets extended to, hopefully, several years. I have never seen the green writing LED light blink on without me specifically triggering it.
I primarily use it as a cronslave [1]. It has ~50 separate cronjobs on it by now, all wheedling away at various things I want to make happen for free on a clock. But if you live out of a terminal and could spend your days happily inside tmux + vim or emacs -nw, there's nothing stopping you from just doing this. Feels a lot like driving stick shift.
I've been running my daily development laptop on 64GB of RAM for 1,5 years. My anecdotal experience is that no, you don't need persistent storage for most things. In fact, often it's in your way -- it clutters the system over time by causing configuration errors and weird undefined program states. When you can just reboot and all works again it's great. Never going back.
I use jupyter notebooks for similar purpose, with the Python kernel's memory keeping the state I want for some random stuff, and notebook being a recipe how to reproduce the calculation/processing.
some of my kernels been running for weeks, as I come back and redo or rework some processing.
the neat thing about jupyter notebooks, is you can interleave python one-liners with bash one-liners and mix them as you wish.
The relevant line from fstab is:
Now any program that writes to /tmp will be writing to a RAM disk, thus sparing unnecessary wear on my SSD.The article has been corrected.
You are relying on random implementation details instead of universal APIs that work across OSes and environments. Please stop.
So help me God, if I make a Linux system, I will make it _not_ have a /dev/shm just to avoid people relying on non-standard stuff for no good reason. Honestly, it's because of stuff like this that we need Docker.
I'm not really seeing a right or wrong here anyway unless you're distributing a script that's meant to run on all sorts of Linux systems. In which case you probably aren't concerned with the physical storage medium being used.
https://pubs.opengroup.org/onlinepubs/9799919799/
It doesn't get more standard than that.
It's because of people doing random nonstandard shit that we need to Docker-ize a lot of software these days. People refuse to lift a single finger to adhere to conventions that let programs co-exist without simulating a whole god damn computational universe for each damn program.
Back in the day you might place /tmp in a good spot for random access of small files on a disk platter. /var is vaguely similar but intended for things that need to be persistent.
Anyway it's not uncommon for systems to persist /tmp and clean it periodically from cron using various retention heuristics.
Ultimately POSIX concepts of mountpoints are strongly tied to optimizing spinning rust performance and maintenance and not necessarily relevant for SSD/NVME.
It doesn't say anything about what it's backed by.
> Usually, it is a better idea to use memory mapped files in /run/ (for system programs) or $XDG_RUNTIME_DIR (for user programs) instead of POSIX shared memory segments, since these directories are not world-writable and hence not vulnerable to security-sensitive name clashes.
$XDG_RUNTIME_DIR usually points to /run/user/${uid}, so you're guaranteed that other users won't write there, and possibly won't even be able to read there.
My current understanding is kernel 2.6, i.e. 2004.
Swap on an SSD isn't even that slow.
Maybe some other ram disk things won't.
This is not the case. RAM-based file system capacities are unrelated to process memory usage, of which "swap space" is for the latter.
I do not run systemd-based distros, so cannot relate.
Glad to help out. Here[0] is more information regarding Linux swap space as it relates to processes and the VMM subsystem.
> I stand by my original point, downvotes be damned.
:-D
0 - https://phoenixnap.com/kb/swap-space
For the author's purposes, any benefit is just placebo.
There absolutely are times where /dev/shm is what you want, but it requires understanding nuances and tradeoffs (e.g. you are already thinking a lot about the memory management going on, including potentially swap).
Don't use -funroll-loops either.
"This optimization [of putting files directly into RAM instead of trusting the buffers] is unnecessary" was an interesting claim, so I decided to put it to the test with `time`.
Compared to the first read there is indeed a large speedup, from 2.2s down to under 0.5s. After the file had been loaded into cache from disk by the first `wc --lines`, however, the difference dropped to /dev/shm being about ~20% faster. Still significant, but not game-changingly so.I'll probably come back to this and run more tests with some of the more complex `jq` query stuff I have to see if we stay at that 20% mark, or if it gets faster or slower.
1 - Programs such as wc (or jq) do sequential reads, which benefit from file systems optimistically prefetching contents in order to reduce read delays.
2 - Check to see if file access time tracking is enabled for the disk-based file system (see mount(8)). This may explain some of the 20% difference.
My use case is to use yt-dlp to download videos to ramfs, watch them and then delete. Before i switched to ramfs, the final pass of yt-dlp (where audio and video tracks are merged to one file) ordinarily caused the issue with choppy system.
Aren't both solved by swapping?
Although I suppose on Linux, neither having swap, nor it being backed by dynamically growing files, is guaranteed.
I've used /dev/shm extensively for large datasets and it's consistently been a massive speed improvement. Not sure what you're talking about.
Well, complain to whoever's mounting it wrong to fix it.
Some hosts should have tmpfs mounted and some shouldn't. For those that don't, I can just /dev/shm. This isn't a "right" or "wrong" sorta thing.
1. I use removable, external drives for anything I want to save long-term. No "cloud" storage.
For the more venturous there is GPURamDrive [1] , not as many options, as it was made as a more of an experiment, but with gpu's adding more and more vram, why not?
[0] https://sourceforge.net/projects/imdisk-toolkit/
[1] https://github.com/abesada/GpuRamDrive
I’m wondering if I can completely hide away the detail where I can work exclusively in memory (even when I habitually save my code) and “reconcile” as some task I do before shutdown.
In fact, that doesn’t even feel necessary… I git push my day’s work a number of times. None of that needs a local disk. And 64GB of memory was surprisingly affordable.
I have it running on a Raspberry Pi so that my already sparingly-used SD card's lifespan gets extended to, hopefully, several years. I have never seen the green writing LED light blink on without me specifically triggering it.
I primarily use it as a cronslave [1]. It has ~50 separate cronjobs on it by now, all wheedling away at various things I want to make happen for free on a clock. But if you live out of a terminal and could spend your days happily inside tmux + vim or emacs -nw, there's nothing stopping you from just doing this. Feels a lot like driving stick shift.
[0]: http://tinycorelinux.net/
[1]: https://hiandrewquinn.github.io/til-site/posts/consider-the-...
some of my kernels been running for weeks, as I come back and redo or rework some processing.
the neat thing about jupyter notebooks, is you can interleave python one-liners with bash one-liners and mix them as you wish.
So in theory some program might pass a name to shm_open that collides with whatever you put in /dev/shm.
Unlikely but possible
Of course, I have always manually configured tmpfs for /tmp/ since Jessie as part of my post-install checklist.