Why is grep fast

No because most blocks will be found in the file system cache. GNU grep is an excellent tool! Give it a try! In my personal tests, it's not at all orders of magnitude faster than GNU grep. In fact, for single file, non-unicode greps most of my usage the difference is so small as to be imperceptible when interactively using it.

This means that compatibility with GNU grep takes priority for me and it's not worth switching over to ripgrep. It's a simple break down of communication. I try to be upfront about this, but no matter how hard I try or how many times I try to clarify it, I can't stop the spread of inaccurate claims. On equivalent tasks, ripgrep is not orders of magnitude faster than GNU grep, outside of pathological cases that involve Unicode support.

I can provide evidence for that if you like. You can imagine that if your directory has a lot of large binary files, or if you're searching in a directory with high latency a network mount , then you might see even bigger differences from ripgrep without generally seeing a difference in search results because ripgrep tends to skip things you don't care about anyway.

In summary, there is an impedance mismatch when talking about performance because most people don't have a good working mental model of how these tools work internally.

Many people report on their own perceived performance improvements and compare that directly to how they used to use grep. They aren't wrong in a certain light, because ultimately, the user experience is what matters. But of course, they are wrong in another light if you're interpreting it as a precise technical claim about the performance characteristics of a program.

You stop looking at entire files, which can avoid a lot of bytes to go through. Also parallelisms surely helps with a lot of files, or big ones really big ones which could be processed in chunks.

No, that's a bad trick to rely on for speed these days. GNU grep's speed comes from using memchr in its skip loop. Skipping bytes is only going to matter when the thing you're searching for is longer. For short needles, there isn't much opportunity for skipping, so it's better to say inside a vectorized routine like memchr which, for glibc, is written in Assembly on most platforms. Whoa, which "time" command are you using that provides memory and page fault info?

Retra on March 29, root parent prev next [—]. Excellent tool. I am using it at work now :. TheGrassyKnoll on March 30, root parent next [—]. Second that. Well done. That situation changes when you have very short patterns, very long patterns, many patterns, or small alphabets eg DNA. So if you cripple ripgrep, to act like grep, it is not much faster. This is kind of obvious. Ripgrep is faster because it makes assumptions, that will hold true for a large part of its users. This is like saying "if I remove wings from a plane, then it won't go much faster than my car.

So a plane is not technically faster than a car" Of course, users should know the difference between grep and ripgrep especially with tweaks like. Yup, that's a known bug. It will be fixed in the next release. I've already done the leg work to fix it, and it should be on master soon. TimTheTinker on March 29, root parent prev next [—]. Scarbutt on March 29, root parent next [—].

This "huge diffrence" is just miliseconds for a 20M LoC, as the author showed in this thread using the linux kernel source, he gives importance to the UX when compared to grep though.

Read the second half of his comment and ask what percentage of the Linux kernel source would be excluded by file type rules versus other projects. I mostly work on web and data archival projects and ripgrep is noticeably seconds or more! For all my searches so far, ripgrep is about x faster. Perhaps you're on some hardware which ripgrep isn't well suited for.

That is simply impossible. You are measuring the wrong thing or a case where rg can skip most files. It's crazy fast. It's more that it uses concurrency intelligently. The underlying stuff is similar to GNU grep. Rust's regex engine uses finite automata, SIMD and aggressive literal optimizations to make searching very fast. Rust's regex library also maintains performance with full Unicode support by building UTF-8 decoding directly into its deterministic finite automaton engine.

The author said in a sister thread[1] that for similar tasks the performance is comparable. Worth mentioning that the main author of ripgrep is also the main author of Rust's regex library, so their development nicely complements each other.

Early on in my career, like most novice programmers, I thought that custom written C programs could be much faster than unix tools if written well and for a specific purpose.

However, I could not beat the speed of unix tools like grep , cut or cat even once. That is when I realized just how well written these tools are and just how much optimization work has been done. It's amazing to me how programs like these have seemed to avoid the universal phenomenon of technical debt. They've crystallized into an ideal version of themselves, and haven't continued to decay past that point.

Maybe it's because of the Unix philosophy of single-purpose programs; no feature-creep tends to mean no technical debt. By keeping a narrow focus and avoiding feature creep, the technical debt has been pretty minimal, and they've been able to pay it down easily. There's at least something to learn from the approach. It does also help that there is a certain type of developer that sees such things as a personal challenge and will go through hell and high water to come up with more and more efficient ways to do things.

There is truth in what you say, however, if you think GNU tools are free of technical debt or feature creep, look into how. Autoconf code is actually quite clean--you just need to know M4 and have an appreciation of the problems autoconf was built to overcome. Also, best practices have shifted to a continuous development model which keeps everybody on the upgrade treadmill. There's less concern with maintaining backwards compatibility and catering to those not running the latest environments.

So if you make use of some newer Linux kernel API there's only a short window where people will put in the effort to maintain a compatibility mode, assuming they bother at all. Lastly, containers mean people often develop and ship static environments that can be maintained independently, sometimes never upgraded at all.

What I find interesting is how people have begun to ditch autoconf in favor of even more complex but newer and therefore cooler build systems when ironically there's less need than ever for these things. Autoconf doesn't need replacing; such layers can often be left out entirely. That said, when feature detection and backwards compatibility truly matters there's no good alternative.

CMake, for example, effectively requires the end user to install the latest version of CMake, and if you already expect someone to install the latest version of something then why the contrivance at all? I always sigh aloud whenever I download a project that relies on CMake because I know that I now have two problems on my hand, not just one.

But better CMake than the other alternatives--I just won't even bother. It's much easier to port to Solaris than AIX. It's a real shame Solaris is disappearing because on big machines with heavy workloads the OOM killer is a fscking nightmare on Linux. Solaris and Windows and maybe AIX? Or even just minutes, because workloads accumulate when the OOM killer starts shooting things down, and even in the cloud you run into hard limits on resource usage--i.

Memory overcommit is just like network buffer bloat--intended to improve things at the small scale but which results in catastrophic degradation at the macro level. A machine can still end up with the OOM killer shooting down processes if, for example, the rate of dirty page generation outpaces the patience of the allocator trying to reclaim and access memory that is technically otherwise available. Well, there's a lot of gross things about CMake the syntax of its DSL is horrifying , but in my experience if you want Windows support it's a lot better than autoconf and make.

However, most gnu stuff is excellent, including gnu make. Fortunately, today most unix systems are sufficiently posix-compliant so that you can ship a gnumakefile that builds your program directly without much fuss. That's romanticizing it a bit It's just more manageable because they've contained the scope of most of the core utilities and packages better than most 'modern' software. Also, don't underestimate the value of having a relatively stable set of maintainers over long periods of time.

How would you do colorization of matches outside of grep, other than reimplementing grep? By optionally emitting generic structural markup which you could then use to format via colorization or whatever other means you wanted in a terminal, a GUI, a web page, etc. That would have been more consistent with the 'one tool, one job' philosophy of Unix. So then you could do something along the lines of 'grep --emit-markup Alternatively, you could have grep take a list of colors from a config file or environment variable and blindly interpolate those strings to make colored output[1].

This also allows color to show up automatically for interactive use. That's not all one might want to do the structured out. Think of converting to HTML, as one example. Of course, ripgrep handles coloring like GNU grep does. I guess what I meant was, most software projects eventually reach a point where they have to be burned down and rebuilt or simply deprecated in favor of a younger project. Depending on the quality of the engineering it could be 5 years or 25, but it feels like an inevitability.

These single-purpose GNU tools seem to be free of that phenomenon. One approach would be to call GNU tools very exceptional and marvel at them keep in mind its origins as a clone of older Unix stuff , but perhaps it's more appropriate to ask tougher questions of people operating in this other modality, that you are calling normal expectations.

If it only does one thing it is a tool. If it does many things it becomes a subordinate with an intelligence of its own, something you need to communicate with or talk to, as opposed to something you can simply use.

I would say it has a fair amount of technical debt. It's possible to beat the speed of cat, actually. See e. One trick is to not free the memory used by the buffer :p. If you're on Linux, you can go way faster by using the splice syscall. It's great to provide links to previous discussions.

Just so everybody knows: the links are for curiosity purposes. For many situations the more complex substring search algorithms gave way to raw, brute force some time ago, I believe. For example, if you are looking for a given relatively short string, you can just take a prefix of say four bytes and then make parallel comparisons within a vector; this simple technique gets you already down to the general area of 1 cpb. The SSE 4.

For short patterns, which are imho by far the most common use, any algorithm that tries to be smart and skip a couple bytes wastes cycles on being smart where a simpler brute force algorithm has already fetched the next 16 bytes and started to compare them while the prefetcher already went off to get the next line from L2. When did you measure SSE4. In my experience this was true more or less never. There's almost nothing in SSE4. Intel has slowly deprecated SSE4. That work was all in the context of searching for a binary string with a mask.

I didn't try too long to optimize it, since performance was quickly satisfactory, so it's not just possible, but rather very likely that my implementation isn't particularly good. I'll keep your advice about SSE 4. I wouldn't use "Teddy" to look for single strings, at least not without heavy modification. The boring approach of hunting for a predicate or two with PCMPEQB or equivalent then shift-and'ing things together has worked well in practice for that sort of thing, although it can be a bit brittle if you get the predicate s wrong.

Ripgrep uses SSE3 parallelization instead of skipping input bytes to get faster on current architectures. I'm assuming it still does both. Parallel is nice. Never touching a byte is tough to beat, though. I've often thought of making sure my IDs are uncommon characters to exploit the ability to skip a lot.

It does not. In particular, the advice in the OP is generally out of date. The "secret" sauce to ripgrep's speed in simple literal searches is a simple heuristic: choose the rarest byte in the needle and feed that to memchr. The "heuristic" is that you can't actually know the optimal choice, but it turns out that a guess works pretty well most of time since most things you search have a similar frequency distribution. The SSSE3 optimizations come from Hyperscan, and are only applicable when searching a small number of small patterns.

In other words, for common searches which are short strings , it is much better to spend more time in a vectorized routine than to try to skip bytes. Complexity analysis of substring search focuses on the number of comparisons — at least those I saw —, much like sorting, and of course that's not an accurate model at all. This kind of thing is an pattern unless the benefit over Boyer-Moore huge. A small performance gain in the common-case is not worth the pain of introducing pathological cases that only bite you once you are deeply committed.

Join Date: Apr If all of those files are in the current directory, then this might work: Code :. Don Cragun. Join Date: Jul If you're looking for a fixed string rather than a match against a regular expression , use grep -F or fgrep depending on which operating system you're using. If you're trying to match the 1st few characters in a file and you have relatively large files and you have a lot of files that won't match, then using read or head to get the start of a file might make sense, but firing up grep and head for every input file is going to be slower than letting grep read the entire file unless the file you're processing is large.

Join Date: Aug If you show what you're doing right now, maybe we can speed it up. Find all posts by Corona I've got 30, files to process using Code :. Try: Code :. It didn't work Corona Grep command giving different result for different users for same command.

Hello, I am running below command as root user nodetool cfstats tests grep "Memtable switch count" Memtable switch count: 12 Where as when I try to run same command as another user it gives different result.

Speed : awk command to count the occurrences of fields from one file present in the other file. Hi, file1. Speed Up Grep. Hi, I have to grep string from 20 - 30 files each carries - MB size and append to the file.

How to speed the grepping time. How to speed up grep? Learn more. Anything faster than grep? Asked 6 years, 10 months ago. Active 6 years, 10 months ago. Viewed 23k times. Improve this question. Jeffrey L. Roberts Jeffrey L. Roberts 1 1 gold badge 5 5 silver badges 22 22 bronze badges.

So you actually plan to search for more than one string, right? Add a comment. Active Oldest Votes. First, you'd have to write a simple shell script that runs grep in the background:! Improve this answer. Kenster Kenster 6, 2 2 gold badges 29 29 silver badges 41 41 bronze badges. If you don't need regex, one benefit of fgrep is you don't have to worry about escaping reserved characters, e. I'm currently running a CPU-bound grep.

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