[31956] in Perl-Users-Digest
Perl-Users Digest, Issue: 3220 Volume: 11
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Fri Nov 26 16:09:22 2010
Date: Fri, 26 Nov 2010 13:09:04 -0800 (PST)
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Fri, 26 Nov 2010 Volume: 11 Number: 3220
Today's topics:
Re: Finding if something is in a list <cartercc@gmail.com>
Re: Memory consumption when writing to files <bart@nijlen.com>
Re: Memory consumption when writing to files <NoSpamPleaseButThisIsValid3@gmx.net>
Posting Guidelines for comp.lang.perl.misc ($Revision: tadmc@seesig.invalid
Digest Administrivia (Last modified: 6 Apr 01) (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: Fri, 26 Nov 2010 11:28:59 -0800 (PST)
From: ccc31807 <cartercc@gmail.com>
Subject: Re: Finding if something is in a list
Message-Id: <2bcf5592-c9c0-495e-acd4-e4d0bb580316@l32g2000yqc.googlegroups.com>
On Nov 25, 2:15=A0pm, "Dave Saville" <d...@invalid.invalid> wrote:
> Well what I think about is all the work the box is doing, making a
> hash and then processing it, keeping the whole file in memory -
> assuming it fits, and thus possibly causing a paging problem, when all
> it needs is a loop reading one record at a time and either passing it
> over or processing it based on a test on what in reality is a pretty
> short list of possible numbers.
I agree that you use the simplest, easiest tool that gets the job
done. If your job requires producing a report from the analysis of two
(or more) input files, you will arrange your work in (at least) three
steps: (1) reading in the data to specific data structures, (2)
processing the data contained in the data structures, and (3) reading
out the processed data to an output file.
One of the great strengths of Perl is that it gives you sophisticated
structures, arrays of hashes, hashes of hashes, hashes of arrays,
etc., nested to arbitrary depths, and doesn't require you to travel
around our elbow to get to your nose.
CC.
------------------------------
Date: Fri, 26 Nov 2010 00:35:25 -0800 (PST)
From: Bart Van der Donck <bart@nijlen.com>
Subject: Re: Memory consumption when writing to files
Message-Id: <a9d99c6e-8d73-444c-932b-90ac455ab52d@o14g2000yqe.googlegroups.com>
Peter J. Holzer wrote:
> On 2010-11-25 10:36, Bart Van der Donck <b...@nijlen.com> wrote:
>
>> =A0 =A0 =A0 #!/usr/bin/perl
>> =A0 =A0 =A0 use strict;
>> =A0 =A0 =A0 use warnings;
>> =A0 =A0 =A0 use Time::HiRes qw(time);
>> =A0 =A0 =A0 print time."\n";
>> =A0 =A0 =A0 open FILE, '>>', 'bigfile' or die $!;
>
> Note: Your headline doesn't match the code. I assume that the ">"
> in the headline is a typo and that the ">>" in the code is correct.
Yes, I accidentally swapped them.
>> =A0 =A0 =A0 print FILE 'newdata';
>> =A0 =A0 =A0 close FILE;
>> =A0 =A0 =A0 print time."\n";
> [...]
>> Average: (26 + 27 + 26 + 25) / 4 =3D 26
>> I am surprised that this operation runs so incredibly fast. My
>> 'bigfile' is 3MB!
>
> 3 MB isn't big. And it (almost) doesn't matter how big the file is,
> since you are only appending a small, fixed amount of data at the end.
> This program probably causes only one write to a data block and one
> write to an inode, and these writes are probably also asynchronous.
>
>> =A0 =A0 =A0 #!/usr/bin/perl
>> =A0 =A0 =A0 use strict;
>> =A0 =A0 =A0 use warnings;
>> =A0 =A0 =A0 use Time::HiRes qw(time);
>> =A0 =A0 =A0 print time."\n";
> > =A0 =A0 =A0 open FILE, '>', c or die $!;
>
> Again, your code doesn't match your headline ('>' vs. '>>') and I'm
> assuming that your code is correct (although the 'c' is suspicious).
Yes, >> should be > and vice versa. 'c' should be 'bigfile'.
>> =A0 =A0 =A0 print FILE 'newdata';
>> =A0 =A0 =A0 close FILE;
>> =A0 =A0 =A0 print time."\n";
> [...]
>> Average: (201 + 195 + 246 + 231) / 4 =3D 218,25
>> Suppose that (1) allocates new memory blocks for 'newdata', and
>> afterwards joins these blocks to the existing blocks of 'bigfile':
>> this could maybe explain the extremely low memory consumption.
>> Suppose that (2) frees the blocks of 'newdata' (truncate action), and
>> then assigns new blocks for 'newdata': this could maybe explain why it
>> is 8.4 times slower.
>
> Yes. If your file is 3 MB at the beginning and your file system uses a
> block size of 4 kB, then the open causes about 750 Blocks to be freed.
> After that a single new block is allocated and written.
>
> Again, all this probably happens asynchronously (the times you reported
> are in 1E-5 seconds, so "218,25" is really 2.1825 ms, which isn't enough
> to write even a single block to a rotating disk), but the OS has to work
> a lot more to trunkate a 3 MB file than to append a few bytes to it.
'>>' =3D take the necessary (few) bytes from the free pool + assign the
content to them ('newdate') + write to an inode so that it's clear
that both ('bigfile' and 'newdata') belong to the same file. Still
pleasantly surprised that the last action doesn't directly deal with
'bigfile'; that must be the reason why eg. Apache's log processing is
so efficient.
'>' =3D truncate 'bigfile' (return these bytes to the free pool) + take
necessary (few) bytes from the free pool + assign the content
('newdate'). The first action has to deal with a lot of blocks, thus
indeed explaining the longer execution time.
Actually this is what I suspected in my first article (see "using >"
vs. "using >>"); good that it's now cleared out.
--
Bart
------------------------------
Date: Fri, 26 Nov 2010 10:25:40 +0100
From: Wolf Behrenhoff <NoSpamPleaseButThisIsValid3@gmx.net>
Subject: Re: Memory consumption when writing to files
Message-Id: <4cef7d15$0$6976$9b4e6d93@newsspool4.arcor-online.net>
On 26.11.2010 09:35, Bart Van der Donck wrote:
> '>>' = take the necessary (few) bytes from the free pool + assign the
> content to them ('newdate') + write to an inode so that it's clear
> that both ('bigfile' and 'newdata') belong to the same file. Still
> pleasantly surprised that the last action doesn't directly deal with
> 'bigfile'; that must be the reason why eg. Apache's log processing is
> so efficient.
Again, this is highly file system dependant!
If blocks have a size of 4k, adding a few bytes to a file with 1 byte
size will write into the very same block. Maybe the storage medium can
only write chunks of some size of data (which might be larger than
"newdata"). In that case, a read needs to be done first. Some other file
systems don't even support appending to a file. And then there is the
cache of the OS which you didn't take into account. You are measuring
the performace of the caching mechanism plus maybe some file system and
disk performance.
I still don't get what this has got to do with Perl and/or memory
consumption.
Cheers, Wolf
------------------------------
Date: Fri, 26 Nov 2010 01:59:53 -0600
From: tadmc@seesig.invalid
Subject: Posting Guidelines for comp.lang.perl.misc ($Revision: 1.9 $)
Message-Id: <t_adncVRp51k9XLRnZ2dnUVZ5oOdnZ2d@giganews.com>
Outline
Before posting to comp.lang.perl.misc
Must
- Check the Perl Frequently Asked Questions (FAQ)
- Check the other standard Perl docs (*.pod)
Really Really Should
- Lurk for a while before posting
- Search a Usenet archive
If You Like
- Check Other Resources
Posting to comp.lang.perl.misc
Is there a better place to ask your question?
- Question should be about Perl, not about the application area
How to participate (post) in the clpmisc community
- Carefully choose the contents of your Subject header
- Use an effective followup style
- Speak Perl rather than English, when possible
- Ask perl to help you
- Do not re-type Perl code
- Provide enough information
- Do not provide too much information
- Do not post binaries, HTML, or MIME
Social faux pas to avoid
- Asking a Frequently Asked Question
- Asking a question easily answered by a cursory doc search
- Asking for emailed answers
- Beware of saying "doesn't work"
- Sending a "stealth" Cc copy
Be extra cautious when you get upset
- Count to ten before composing a followup when you are upset
- Count to ten after composing and before posting when you are upset
-----------------------------------------------------------------
Posting Guidelines for comp.lang.perl.misc ($Revision: 1.9 $)
This newsgroup, commonly called clpmisc, is a technical newsgroup
intended to be used for discussion of Perl related issues (except job
postings), whether it be comments or questions.
As you would expect, clpmisc discussions are usually very technical in
nature and there are conventions for conduct in technical newsgroups
going somewhat beyond those in non-technical newsgroups.
The article at:
http://www.catb.org/~esr/faqs/smart-questions.html
describes how to get answers from technical people in general.
This article describes things that you should, and should not, do to
increase your chances of getting an answer to your Perl question. It is
available in POD, HTML and plain text formats at:
http://www.rehabitation.com/clpmisc.shtml
For more information about netiquette in general, see the "Netiquette
Guidelines" at:
http://andrew2.andrew.cmu.edu/rfc/rfc1855.html
A note to newsgroup "regulars":
Do not use these guidelines as a "license to flame" or other
meanness. It is possible that a poster is unaware of things
discussed here. Give them the benefit of the doubt, and just
help them learn how to post, rather than assume that they do
know and are being the "bad kind" of Lazy.
A note about technical terms used here:
In this document, we use words like "must" and "should" as
they're used in technical conversation (such as you will
encounter in this newsgroup). When we say that you *must* do
something, we mean that if you don't do that something, then
it's unlikely that you will benefit much from this group.
We're not bossing you around; we're making the point without
lots of words.
Do *NOT* send email to the maintainer of these guidelines. It will be
discarded unread. The guidelines belong to the newsgroup so all
discussion should appear in the newsgroup. I am just the secretary that
writes down the consensus of the group.
Before posting to comp.lang.perl.misc
Must
This section describes things that you *must* do before posting to
clpmisc, in order to maximize your chances of getting meaningful replies
to your inquiry and to avoid getting flamed for being lazy and trying to
have others do your work.
The perl distribution includes documentation that is copied to your hard
drive when you install perl. Also installed is a program for looking
things up in that (and other) documentation named 'perldoc'.
You should either find out where the docs got installed on your system,
or use perldoc to find them for you. Type "perldoc perldoc" to learn how
to use perldoc itself. Type "perldoc perl" to start reading Perl's
standard documentation.
Check the Perl Frequently Asked Questions (FAQ)
Checking the FAQ before posting is required in Big 8 newsgroups in
general, there is nothing clpmisc-specific about this requirement.
You are expected to do this in nearly all newsgroups.
You can use the "-q" switch with perldoc to do a word search of the
questions in the Perl FAQs.
Check the other standard Perl docs (*.pod)
The perl distribution comes with much more documentation than is
available for most other newsgroups, so in clpmisc you should also
see if you can find an answer in the other (non-FAQ) standard docs
before posting.
It is *not* required, or even expected, that you actually *read* all of
Perl's standard docs, only that you spend a few minutes searching them
before posting.
Try doing a word-search in the standard docs for some words/phrases
taken from your problem statement or from your very carefully worded
"Subject:" header.
Really Really Should
This section describes things that you *really should* do before posting
to clpmisc.
Lurk for a while before posting
This is very important and expected in all newsgroups. Lurking means
to monitor a newsgroup for a period to become familiar with local
customs. Each newsgroup has specific customs and rituals. Knowing
these before you participate will help avoid embarrassing social
situations. Consider yourself to be a foreigner at first!
Search a Usenet archive
There are tens of thousands of Perl programmers. It is very likely
that your question has already been asked (and answered). See if you
can find where it has already been answered.
One such searchable archive is:
http://groups.google.com/advanced_search
If You Like
This section describes things that you *can* do before posting to
clpmisc.
Check Other Resources
You may want to check in books or on web sites to see if you can
find the answer to your question.
But you need to consider the source of such information: there are a
lot of very poor Perl books and web sites, and several good ones
too, of course.
Posting to comp.lang.perl.misc
There can be 200 messages in clpmisc in a single day. Nobody is going to
read every article. They must decide somehow which articles they are
going to read, and which they will skip.
Your post is in competition with 199 other posts. You need to "win"
before a person who can help you will even read your question.
These sections describe how you can help keep your article from being
one of the "skipped" ones.
Is there a better place to ask your question?
Question should be about Perl, not about the application area
It can be difficult to separate out where your problem really is,
but you should make a conscious effort to post to the most
applicable newsgroup. That is, after all, where you are the most
likely to find the people who know how to answer your question.
Being able to "partition" a problem is an essential skill for
effectively troubleshooting programming problems. If you don't get
that right, you end up looking for answers in the wrong places.
It should be understood that you may not know that the root of your
problem is not Perl-related (the two most frequent ones are CGI and
Operating System related), so off-topic postings will happen from
time to time. Be gracious when someone helps you find a better place
to ask your question by pointing you to a more applicable newsgroup.
How to participate (post) in the clpmisc community
Carefully choose the contents of your Subject header
You have 40 precious characters of Subject to win out and be one of
the posts that gets read. Don't waste them. Take care while
composing them, they are the key that opens the door to getting an
answer.
Spend them indicating what aspect of Perl others will find if they
should decide to read your article.
Do not spend them indicating "experience level" (guru, newbie...).
Do not spend them pleading (please read, urgent, help!...).
Do not spend them on non-Subjects (Perl question, one-word
Subject...)
For more information on choosing a Subject see "Choosing Good
Subject Lines":
http://www.cpan.org/authors/id/D/DM/DMR/subjects.post
Part of the beauty of newsgroup dynamics, is that you can contribute
to the community with your very first post! If your choice of
Subject leads a fellow Perler to find the thread you are starting,
then even asking a question helps us all.
Use an effective followup style
When composing a followup, quote only enough text to establish the
context for the comments that you will add. Always indicate who
wrote the quoted material. Never quote an entire article. Never
quote a .signature (unless that is what you are commenting on).
Intersperse your comments *following* each section of quoted text to
which they relate. Unappreciated followup styles are referred to as
"top-posting", "Jeopardy" (because the answer comes before the
question), or "TOFU" (Text Over, Fullquote Under).
Reversing the chronology of the dialog makes it much harder to
understand (some folks won't even read it if written in that style).
For more information on quoting style, see:
http://web.presby.edu/~nnqadmin/nnq/nquote.html
Speak Perl rather than English, when possible
Perl is much more precise than natural language. Saying it in Perl
instead will avoid misunderstanding your question or problem.
Do not say: I have variable with "foo\tbar" in it.
Instead say: I have $var = "foo\tbar", or I have $var = 'foo\tbar',
or I have $var = <DATA> (and show the data line).
Ask perl to help you
You can ask perl itself to help you find common programming mistakes
by doing two things: enable warnings (perldoc warnings) and enable
"strict"ures (perldoc strict).
You should not bother the hundreds/thousands of readers of the
newsgroup without first seeing if a machine can help you find your
problem. It is demeaning to be asked to do the work of a machine. It
will annoy the readers of your article.
You can look up any of the messages that perl might issue to find
out what the message means and how to resolve the potential mistake
(perldoc perldiag). If you would like perl to look them up for you,
you can put "use diagnostics;" near the top of your program.
Do not re-type Perl code
Use copy/paste or your editor's "import" function rather than
attempting to type in your code. If you make a typo you will get
followups about your typos instead of about the question you are
trying to get answered.
Provide enough information
If you do the things in this item, you will have an Extremely Good
chance of getting people to try and help you with your problem!
These features are a really big bonus toward your question winning
out over all of the other posts that you are competing with.
First make a short (less than 20-30 lines) and *complete* program
that illustrates the problem you are having. People should be able
to run your program by copy/pasting the code from your article. (You
will find that doing this step very often reveals your problem
directly. Leading to an answer much more quickly and reliably than
posting to Usenet.)
Describe *precisely* the input to your program. Also provide example
input data for your program. If you need to show file input, use the
__DATA__ token (perldata.pod) to provide the file contents inside of
your Perl program.
Show the output (including the verbatim text of any messages) of
your program.
Describe how you want the output to be different from what you are
getting.
If you have no idea at all of how to code up your situation, be sure
to at least describe the 2 things that you *do* know: input and
desired output.
Do not provide too much information
Do not just post your entire program for debugging. Most especially
do not post someone *else's* entire program.
Do not post binaries, HTML, or MIME
clpmisc is a text only newsgroup. If you have images or binaries
that explain your question, put them in a publically accessible
place (like a Web server) and provide a pointer to that location. If
you include code, cut and paste it directly in the message body.
Don't attach anything to the message. Don't post vcards or HTML.
Many people (and even some Usenet servers) will automatically filter
out such messages. Many people will not be able to easily read your
post. Plain text is something everyone can read.
Social faux pas to avoid
The first two below are symptoms of lots of FAQ asking here in clpmisc.
It happens so often that folks will assume that it is happening yet
again. If you have looked but not found, or found but didn't understand
the docs, say so in your article.
Asking a Frequently Asked Question
It should be understood that you may have missed the applicable FAQ
when you checked, which is not a big deal. But if the Frequently
Asked Question is worded similar to your question, folks will assume
that you did not look at all. Don't become indignant at pointers to
the FAQ, particularly if it solves your problem.
Asking a question easily answered by a cursory doc search
If folks think you have not even tried the obvious step of reading
the docs applicable to your problem, they are likely to become
annoyed.
If you are flamed for not checking when you *did* check, then just
shrug it off (and take the answer that you got).
Asking for emailed answers
Emailed answers benefit one person. Posted answers benefit the
entire community. If folks can take the time to answer your
question, then you can take the time to go get the answer in the
same place where you asked the question.
It is OK to ask for a *copy* of the answer to be emailed, but many
will ignore such requests anyway. If you munge your address, you
should never expect (or ask) to get email in response to a Usenet
post.
Ask the question here, get the answer here (maybe).
Beware of saying "doesn't work"
This is a "red flag" phrase. If you find yourself writing that,
pause and see if you can't describe what is not working without
saying "doesn't work". That is, describe how it is not what you
want.
Sending a "stealth" Cc copy
A "stealth Cc" is when you both email and post a reply without
indicating *in the body* that you are doing so.
Be extra cautious when you get upset
Count to ten before composing a followup when you are upset
This is recommended in all Usenet newsgroups. Here in clpmisc, most
flaming sub-threads are not about any feature of Perl at all! They
are most often for what was seen as a breach of netiquette. If you
have lurked for a bit, then you will know what is expected and won't
make such posts in the first place.
But if you get upset, wait a while before writing your followup. I
recommend waiting at least 30 minutes.
Count to ten after composing and before posting when you are upset
After you have written your followup, wait *another* 30 minutes
before committing yourself by posting it. You cannot take it back
once it has been said.
AUTHOR
Tad McClellan and many others on the comp.lang.perl.misc newsgroup.
--
Tad McClellan
email: perl -le "print scalar reverse qq/moc.liamg\100cm.j.dat/"
The above message is a Usenet post.
I don't recall having given anyone permission to use it on a Web site.
------------------------------
Date: 6 Apr 2001 21:33:47 GMT (Last modified)
From: Perl-Users-Request@ruby.oce.orst.edu (Perl-Users-Digest Admin)
Subject: Digest Administrivia (Last modified: 6 Apr 01)
Message-Id: <null>
Administrivia:
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End of Perl-Users Digest V11 Issue 3220
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