[Solved] How can I keep cargo build artifacts in RAM? (programming.dev)

submitted 6 days ago* (last edited 6 days ago) by onlinepersona@programming.dev to c/rust@programming.dev

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I've got 64 GB of RAM and would like to force cargo to dump build artifacts into it. So basically the target/ directory should end up there.

Unless I'm mistaken RAM is much faster than SSDs and since I do rebuild quite often, it would save some R/W cycles on my SSD and allow faster file access.

I do jot mind doing a full rebuild every morning

Solution:

These 2 comments gave me the best indication how to do it: cargo ramdisk and build.target-dir config options.

Would be great if cargo had a build.target-dir-prefix though. One could set and env var CARGO_TARGET_DIR_PREFIX and point it at /dev/shm or /tmp if it's a tmpfs and every rust project would have its artefacts end up in RAM.

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[-] deadcream@sopuli.xyz 28 points 6 days ago

On Linux you can use tmpfs

[-] MoSal@lemm.ee 7 points 6 days ago

/tmp is still tmpfs here, but I prefer this (below) nowadays for usages that are not typical /tmp usages:

  n=$(cat /sys/class/zram-control/hot_add)
  echo zstd > /sys/block/zram${n}/comp_algorithm # change that if you like
  echo 8G > /sys/block/zram${n}/disksize # larger disk probably needed for /target
  mkfs.ext4 -O '^has_journal' -L TMPZ -m 0.001 /dev/zram${n}
  # discard is important for auto-trimming
  mount -o noatime,discard /dev/zram${n} /tmpz
  chmod 777 /tmpz
  chmod +t /tmpz
  rm -rf '/tmpz/lost+found'

This is a part of my zram devices initialization script, a bunch used for swap as usual, then ending with this. modprobe zram num_devices=0 works if one likes to hot_add all devices.

From zram you get a block device with builtin compression. From ext4, you still get features like creation time, truncate and fallocate support, ...etc. And with discard, ram usage will be limited to used space (like tmpfs). Also ext4 is used without a journal to avoid what would be useless overhead in this use-case.

[-] ede1998@feddit.org 16 points 6 days ago

As other already mentioned you can mount a RAM disk yourself to where the target folder would be placed. Alternatively you can change where cargo places its artifacts to somewhere you already have a ram disk mounted, e.g. /tmp

https://doc.rust-lang.org/cargo/reference/config.html#buildtarget-dir

[-] stsquad@lemmy.ml 8 points 6 days ago

Your build artefacts will already be in RAM as the kernel will utilise any spare for the block cache. The block cache is evicted using a least recently used (LRU) basis when the system is under memory pressure. Using ram based filesystems is basically bypassing the kernel and reducing it's flexibility to make the best use of spare RAM in the system.