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posted by martyb on Wednesday December 16 2015, @06:23AM   Printer-friendly
from the it-all-adds-up dept.

Okay, maybe not everything you know about latency is wrong. But now that I have your attention, we can talk about why the tools and methodologies you use to measure and reason about latency are likely horribly flawed. In fact, they're not just flawed, they're probably lying to your face.

When I went to Strange Loop in September, I attended a workshop called "Understanding Latency and Application Responsiveness" by Gil Tene. Gil is the CTO of Azul Systems, which is most renowned for its C4 pauseless garbage collector and associated Zing Java runtime. While the workshop was four and a half hours long, Gil also gave a 40-minute talk called "How NOT to Measure Latency" which was basically an abbreviated, less interactive version of the workshop. If you ever get the opportunity to see Gil speak or attend his workshop, I recommend you do. At the very least, do yourself a favor and watch one of his recorded talks or find his slide decks online.

The remainder of this [linked] post is primarily a summarization of that talk. You may not get anything out of it that you wouldn't get out of the talk, but I think it can be helpful to absorb some of these ideas in written form. Plus, for my own benefit, writing about them helps solidify it in my head.


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  • (Score: 2) by Fnord666 on Wednesday December 16 2015, @03:23PM

    by Fnord666 (652) on Wednesday December 16 2015, @03:23PM (#277128) Homepage
    From TFA:

    The median is the number that 99.9999999999% of response times will be worse than. This is why median latency is irrelevant.

    Wait, what?

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  • (Score: 0) by Anonymous Coward on Wednesday December 16 2015, @05:08PM

    by Anonymous Coward on Wednesday December 16 2015, @05:08PM (#277204)

    you missed the preamble.

    IF a request is considered satisfied when the final result is back from 40 independent sub-requests whose latencies all share the same distribution, then the median is ... (take the log2 of 1 - his-number, that will tell you what what the "40" really is, I eyeballed)

    However, this is equivocation - his "request" is both the outer request and the inner request.

    He also gets a bit anti- means. His claim that a mean of a percentile is meaningless is no more valid than saying that the mean of anything is meaningless. It's not a percentile, it's an approximation to a likely expected percentile, in the same way that the average amount of drink consumed on a booze cruise will be the amount of drink consumed by someone on a booze cruise. Averages are just averages, get over it, they are neither meaningless nor wrong.

    FatPhil (AC as browsing on his phone, while incidentally in the karaoke of a booze cruise ship surrounded by drunk Finns - or "Finns" as I like to call them, our four-legged northern neighbours.)

    • (Score: 0) by Anonymous Coward on Wednesday December 16 2015, @05:39PM

      by Anonymous Coward on Wednesday December 16 2015, @05:39PM (#277218)

      Ooops, karaoke is doing my nut in... s/will be the amount/will *not* be the amount/

      FFFFFUUUUUU!!!!!!!! now in the cafe surrounded by noisy little crack monkeys...