jboner

Latency Comparison Numbers (~2012)
----------------------------------
L1 cache reference                           0.5 ns
Branch mispredict                            5   ns
L2 cache reference                           7   ns                      14x L1 cache
Mutex lock/unlock                           25   ns
Main memory reference                      100   ns                      20x L2 cache, 200x L1 cache
Compress 1K bytes with Zippy             3,000   ns        3 us
Send 1K bytes over 1 Gbps network       10,000   ns       10 us
Read 4K randomly from SSD*             150,000   ns      150 us          ~1GB/sec SSD

djspiewak

Thread Pools

Thread pools on the JVM should usually be divided into the following three categories:

1. CPU-bound
2. Blocking IO
3. Non-blocking IO polling

Each of these categories has a different optimal configuration and usage pattern.

mobilemind

# delete local tag '12345'
git tag -d 12345
# delete remote tag '12345' (eg, GitHub version too)
git push origin :refs/tags/12345
# alternative approach
git push --delete origin tagName
git tag -d tagName

djspiewak

Quick Tips for Fast Code on the JVM

I was talking to a coworker recently about general techniques that almost always form the core of any effort to write very fast, down-to-the-metal hot path code on the JVM, and they pointed out that there really isn't a particularly good place to go for this information. It occurred to me that, really, I had more or less picked up all of it by word of mouth and experience, and there just aren't any good reference sources on the topic. So… here's my word of mouth.

This is by no means a comprehensive gist. It's also important to understand that the techniques that I outline in here are not 100% absolute either. Performance on the JVM is an incredibly complicated subject, and while there are rules that almost always hold true, the "almost" remains very salient. Also, for many or even most applications, there will be other techniques that I'm not mentioning which will have a greater impact. JMH, Java Flight Recorder, and a good profiler are your very best friend! Mea

djspiewak

Fibers

Fibers are an abstraction over sequential computation, similar to threads but at a higher level. There are two ways to think about this model: by example, and abstractly from first principles. We'll start with the example.

(credit here is very much due to Fabio Labella, who's incredible Scala World talk describes these ideas far better than I can)

Callback Sequentialization

Consider the following three functions

EncodePanda

Since macOS Catalina, the root drive is read-only. The solution is to create a separate APFS volume and a “synthetic” /nix directory which points to it:

# Check if /nix exists, if not:
echo 'nix' | sudo tee -a /etc/synthetic.conf
# this will create a "synthetic" empty directory /nix

# REBOOT so macOS sees the synthetic directory

# After rebooting, create an APFS volume for Nix

danbst

Okay, I've got a need to build Firefox from source, and I'd like to do that on a remote machine, and then copy build result back to my laptop. With Nix, using bastion host. I'll note details of my successful adventure.

Setup & Sources of knowledge

Here's the list of resources I've used actively:

• https://nixos.wiki/wiki/Distributed_build
• https://nixos.org/nix/manual/#conf-trusted-substituters and various other configuration parameters
• https://github.com/NixOS/nixpkgs/blob/release-19.09/nixos/modules/services/misc/nix-daemon.nix which contains mappings from Nix config names to NixOS config names (damn, they are different!)

Here's my setup:

Daenyth

package teikametrics
import java.nio.charset.{Charset, StandardCharsets}

import akka.util.ByteString
import fs2.{Chunk, Pipe, RaiseThrowable, Stream}
import teikametrics.ByteEncoding._

/** Streamed bytes claiming to be encoded under `BE`
  *
  * @param chunkSize A chunk size to use for streaming operations, in order to use a consistent value and minimize array copying

paulp

// These lines go in ~/.sbt/0.13/global.sbt
watchSources ++= (
     (baseDirectory.value * "*.sbt").get
  ++ (baseDirectory.value / "project" * "*.scala").get
  ++ (baseDirectory.value / "project" * "*.sbt").get
)

addCommandAlias("rtu", "; reload ; test:update")
addCommandAlias("rtc", "; reload ; test:compile")
addCommandAlias("ru", "; reload ; update")