[erlang-questions] Strings and Text Processing

Dmitry Kolesnikov dmkolesnikov@REDACTED
Sat Dec 29 17:21:56 CET 2012

Hello Joe,

I do re-call you strategy "keep large strings as binaries when I'm not working on them". I was trying to reference it in my previous email :-) but did not find a link to it.

One thing, what I am not fully agree with you is a statement about memory on modern machines… 
Let me give you a simple example about EC2 cloud:
* M1 Small Instance (Default) 1.7 GiB of memory
* M1 Medium Instance 3.75 GiB of memory
* M1 Large Instance 7.5 GiB of memory

So I am using EC2 to run my service for consumers, I have to keep 4KB of json objects per registered user.
Using lists, "Each character consumes 8 bytes of memory on a 32 bit machine…". 4KB data blows up 8x times.
Taking into assumption that active users are kept in memory, each node handles
* M1 Small Instance (Default) 1.7 GiB of memory --> 53K active users
* M1 Medium Instance 3.75 GiB of memory --> 117K active users
* M1 Large Instance 7.5 GiB of memory -->  234K active users
Using binaries same metrics could be improved 8x times.

Note: the following calculation is artificial. It just demonstrates that memory metrics is really depends on use-case.

BTW, memory utilisation was a reason why I moved from mochijson to jsx, it parses json as binary. The drop in performance was acceptable but memory consumption was improved 2-3 times...

All in-all, I do agree that string are changes during processing, use-case, etc.

- Dmitry

On Dec 29, 2012, at 5:30 PM, Joe Armstrong <erlang@REDACTED> wrote:

> On Sat, Dec 29, 2012 at 3:20 PM, Dmitry Kolesnikov <dmkolesnikov@REDACTED> wrote:
> Hello Steve,
> You have raised a good point here.
> One more reason for binary is memory consumption and IPC overhead.
> The point about memory consumption is raised *many* times - on a modern
> machine this is not a problem.
> Example: I am working on a text file of 84KB - in a 32 bit Erlang we use
> 8 bytes/character - so I use 0.6 MB - I have 4GB memory - so I use 0.015% of
> memory - ie no problem.
> My strategy is to keep large strings as binaries when I'm not working on them,
> turn them into lists in order to work on them, and turn them back into binaries
> when I'm done. Just because a string starts off in a binary does not mean
> that it has to stay as a binary as you work on it.
> Imagine I have a lot of text files, say each of 50KB, I can store 20 per/MB or
> 20,000 files per GB. Assume I have a quad core. I can only work on four things
> at the same time - so having (say) 20,000 files (at 50K) and work on four of them
> (unpacked) at a time is another 1.6 Meg.
> Gigabyte memories mean (among other things) what saving the odd byte here are there is hardly relevant.
> On another hand list allows to represent a code point per element.
> yes - the convenience of having one character per list element far outweighs
> the space saving of storing strings in binaries
> iolists are also very handy to dynamically compose a complex strings.
> I am afraid that this is an application specific questions… However, I tend to use binary for strings...
> My strings change form depending on what I'm doing. Sometimes they are
> binaries, sometimes lists, sometimes trees, ...
> Cheers
> /Joe
> - Dmitry
> On Dec 29, 2012, at 4:08 PM, Steve Davis <steven.charles.davis@REDACTED> wrote:
> > Disclaimer :-) All the below is prefixed by a big IMHO
> >
> > Erlang has been correctly criticized for the difficulty of handling "strings".
> >
> > There are two reasons for this (fundamental decisions that were taken way-back-when):
> > 1) "strings" are "just lists of integers"
> > 2) "strings" are by default latin-1 representations
> >
> > This introduces major inconveniences, some of which are not resolvable:
> > When faced with any list during pattern matching, it is not at all easy to determine whether that list is a "string".
> > Further, since strings are "only" a subset of the set of lists of integers, it can be impossible to determine programmatically whether the list is a list of integers or is meant to represent a string. Determining whether a particular list even qualifies as a string in a program requires non-trivial processing of the entire list.
> >
> > It's rather unfortunate that Erlang has earned this reputation, since the truth is that Erlang is truly excellent at text processing. However, to benefit from this excellence, you need to do two things:
> > 1) Represent and process text as binaries.
> > 2) Assume that the text binary is UTF-8 encoded, unless otherwise stated (meaning, e.g. #text{encoding = cstring, value = <<116,101,120,116,0>>}).
> >
> > Suddenly, thanks to binary syntax and pattern matching, processing text in your programs becomes deterministic and easy. (Note that part of the reason for this is that binaries are "expected" to be opaque, whereas general list processing is fundamental to writing any program in Erlang).
> >
> > There's a couple of minor drawbacks, both of which are the result of the initial decisions about "strings":
> > 1) The code is littered with additional angle brackets <<"string">> (annoying, but definitely worth the inconvenience)
> > 2) The standard Erlang/OTP library functions require textual arguments as lists (requiring overuse of binary_to_list)
> >
> > And there are further benefits:
> > 1) Parsing/transcoding different charset encodings is far more straightforward
> > 2) Internationalization/localization is far more straightfoward
> >
> > I wonder if, had the current binary pattern matching/comprehensions been available "way-back-when", whether the decision about "string" representation in Erlang may have been different. (i.e. <<116,101,120,116>> = "text").
> >
> > Finally, here's my two questions:
> > 1) Is there any benefit at all to the "list representation" of strings above binary text?
> > 2) If not, I wonder if there's any way to change our minds about "strings" as we enter 2013?
> >
> > regs,
> > /s
> >
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