5 ms·
Nice examples and graphs. What really confuses me is the definition of "low-latency" nowadays. The meaning suffers a slippery slope in recent years. It used to
by htfy96 8y ago
Nice examples and graphs. What really confuses me is the definition of "low-latency" nowadays. The meaning suffers a slippery slope in recent years. It used to refer to a scale of microseconds at HFT shops, then it came to web request latency at a scale of milliseconds. Now every GC-based language claims they are "low-latency" because 90%/95%/99% GC stop is within 16.67ms/50ms/whatever. Therefore today some HFT developers invented words like "ultra-low latency"[0] to name their work.
[0]: https://en.wikipedia.org/wiki/Ultra-low_latency_direct_market_access https://en.wikipedia.org/wiki/Ultra-low_latency_direct_marke...
- aidenn0 8y agoPrior to HFT shops, embedded use cases might consider 100s of us to be the threshold for low-latency (40 instructions on a typical 8-bit micro running in single-digit MHz is under 100us, with no cache-misses to muck up timings).
- jacobolus 8y ago> What really confuses me is the definition of "low-latency" nowadays There has never been a strict definition of “low latency”. It is heavily context-dependent. Something which is not “low latency” enough for live audio or a car’s steering controller might be plenty fast for an internet text chat service.
- gpderetta 8y agoA better term is soft or hard real-time.
- baragiola 8y agoThat's been the case for operating systems and it's quite effective
- zokier 8y agoWhile not completely orthogonal, I feel like real-time (soft or hard) is distinct from low latency; real-time can be still relatively high-latency as long as the latency is bounded/controlled/predictable. Of course generally in practice real-time systems strive to have low latency too.
- jacobush 8y agoAbsolutely. Think about turning a super tanker or something other huge. It must happen on time. It doesn't matter if it takes one or two seconds to initiate.
- true_religion 8y agoI thought the difference between soft vs hard realtime wasn't one of magnitude, but context. Soft: After X time, usability of the data degrades with age. Hard: After X time, the data is useless (example: realtime car control systems; if you take too long the car has already crashed)
- gpderetta 8y agoOh, sorry, I mean that low latency is a synonym of real-time, either hard or soft depending on the application.
- TickleSteve 8y agoIts related, but not equivalent. real-time == "bounded latency" (to a soft or hard degree). low-latency implies no threshold, just low relative to something else.
- spamizbad 8y agoNot exactly. Hard Real-Time = Missing a deadline is a major system fault. Hard read-time would be anti-lock breaks on your car: Did the cylinder not actuate within a given deadline? If so, disable the ABS feature system since its no longer deterministic, set an error code on the car's computer, and make some error light on the dashboard go on. Soft real time is video decoding: We need to render 1 frame every 1/30th-ish per second. If we miss our deadline, we either need to skip that frame and move onto the next, or pause to fill a buffer. You can have a hard real time system where the latency required is measured in seconds and a soft realtime system where the deadline is in nanoseconds.
- CoolGuySteve 8y agoSomeone at Google once told me they worked on “real-time” search. Having come from graphics and video rendering I asked what that meant and she said it was how search may or may not respond to key presses. Whatever it was, it certainly wasn’t real-time.
- steventhedev 8y agoContext is king. When you're working on the timescale of days, low latency can be within minutes. By the same measure, when you expect to get a new event every microsecond, then nanoseconds matter. In this case, Gambit (the outfit who sponsored this work) are a stat arb shop doing HFT. So I'd say the label fits.
- aldanor 8y agoIn HFT shops, we usually categorise stuff as “hardware” (fpga) and “software” these days. Low-latency typically means sub-microsecond.
- walterbell 8y agoDo HFT shops use open-source code for FPGA dev?
- aldanor 8y agoI’d say typically not, there’s not much to use as most of the FPGA code would be know-how and optimised down to cycle-level for each particular exchange.
- walterbell 8y agoHow about basic frameworks like high-speed PCIe communication with the operating system, including an open-source driver for Windows and Linux? That should be algo independent.
- aldanor 8y agoI’m not an fpga engineer myself in my firm, so I can’t answer that with certainty, but one thing I know for sure is how strong the “not invented here” syndrome is in this kind of shops, so you can guess...
- walterbell 8y agoA bit of competition is always fun, you can point them at the code/papers linked from https://forums.xilinx.com/t5/Xcell-Daily-Blog-Archived/Need-fast-efficient-open-source-PCIe-DMA-for-your-FPGA-RIFFA-2-2/ba-p/642947 https://forums.xilinx.com/t5/Xcell-Daily-Blog-Archived/Need-...
- mcpherrinm 8y agoOn the flip side, I've also heard "low-latency" (or even "instant") to mean "within 30 seconds", as an upgrade from "the mainframe runs a batch job every hour (or day)". I don't think there's any way to consider a phrase like "low latency" without considering what you're talking about.
- milkytron 8y agoGood point, the word "low" itself is relevant.
- SEJeff 8y agoHFT shops refer to things on nanosecond and sometimes even picosecond scale if you want to talk seriously about it. Source: software engineer who's worked (or still works) for electronic trading firms for the last 10ish years.
- joshuamorton 8y agoI'd really like to know the context in which the time for light to travel ~1 inch or (generously) 1/200 of 1 cpu operation or 1/100 of a cache read is relevant to anything.
- shaklee3 8y agoFor RF work it's common to have to send a signal at a nanosecond resolution to be received properly due to time sharing.
- SEJeff 8y agoThe more accurate data captures are, the more accurate models of the data can become. Also, for things like pure arbitrage (which virtually doesn't exist anymore due to the efficiency of markets with computers trading), those trades are Taladega Nights style "you're first or you're last" winner take all. As another poster commented about, it is not uncommon in RF. Fiber optics along with Microwave radios are common in the space. The goal is to get to the limit of what physics allows ultimately.