We are seeing significantly higher latency when calling System.cmd from an event in a LiveView vs when the LiveView first loads. In order to demonstrate this cleanly (our app is fairly complex) I created an entirely new 1.7.2 project with mix phx.new ptest --no-ecto and added the following to lib/ptest_web/live/ls_live/index.ex.
defmodule PtestWeb.LsLive.Index do
use PtestWeb, :live_view
def mount(_params, _session, socket) do
{:ok, assign(socket, :files, get_files())}
end
def render(assigns) do
~H"""
<button phx-click="refresh">refresh</button>
<div :for={file <- @files}><%= file %></div>
"""
end
def handle_event("refresh", _params, socket) do
{:noreply, assign(socket, :files, get_files())}
end
defp get_files() do
timer = Time.utc_now()
{files, _} = System.cmd("ls", [])
IO.puts("System.cmd took #{Time.diff(Time.utc_now(), timer, :milliseconds)} milliseconds")
String.split(files, "\n")
end
end
And, of course, the route: live "/ls", LsLive.Index, :index.
That System.cmd takes 2 milliseconds on average when called from mount on page load vs 15.6 milliseconds on average when called from handle_event when clicking the button. In our app we see about a 3x increase from ~11 millisecond when we call System.cmd from mount to ~33 when called from handle_event.
Any ideas why?
The file listing was just to demonstrate the issue. You can swap that ls for any other shell command and see the same issue. You can even see the same issue with File.ls, it’s just the numbers are much smaller because File.ls is, as you say, much faster. The issue is the same though:
