Comment by marginalia_nu | Hacker News Reader

Comment by marginalia_nu | Hacker News Reader

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I'm no fan of journald, but I have some methodological issues with this test.

The reads from /tmp/all.log are almost certainly cached since you just wrote the file, and will basically boil down to a memcpy call, rather than actual disk I/O. Speed difference isn't as big as you would think on a modern SSD, but it isn't nothing either.

Running this between calls should flush the changes to disk and then drop the page cache, making for a fairer test.

$ sudo sync

$ echo 3 | sudo tee /proc/sys/vm/drop_caches

I tried similar. Each command looped 5 times, ignore the best and worst times, and average the mean three.

    journalctl >/tmp/all.log  0m59.763s
    journalctl | grep -c sshd 0m57.767s  (otherwise >all is the only example encumbered by write speeds)
    wc -l /tmp/all.log        0m0.336s
    grep -c sshd /tmp/all.log 0m1.776s

You didn't use the query feature of journalctl, so you basically had it do an entire table conversion. Of course that's not going to show its performance value.

This is what you should be running for a proper comparison:

1. `echo 3 | sudo tee /proc/sys/vm/drop_caches`

2. `time journalctl -u ssh.service >/dev/null`

3. `journalctl >/tmp/all.log`

4. `echo 3 | sudo tee /proc/sys/vm/drop_caches`

5. `time grep -q sshd /tmp/all.log`

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