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Re^3: selecting columns from a tab-separated-values file

by BrowserUk (Pope)
on Jan 22, 2013 at 17:15 UTC ( #1014738=note: print w/replies, xml ) Need Help??


in reply to Re^2: selecting columns from a tab-separated-values file
in thread selecting columns from a tab-separated-values file

taking care that I/O doesnít sneak up on you from the backside in the form of virtual-memory paging.

You and your hobby horses. Virtual memory limits don't enter into it.

When running, those two programs I posted use 2.7 MB and 3.2 MB respectively when using the 256 times larger read size than standard that I suggest. Even if I increase that 10-fold to 10MB each -- which slows the processing down -- they use a whole 12 MB & 16 MB. The programmer that runs my heating system could handle that.

Iím personally not sure that threads would help here

I'm not sure that standing on one leg whilst drinking blood from a freshly decapitated chicken would help; so I don;t mention it.

And your instincts prejudiced guessing is wrong!

Approximately 2/3rds of the throughput gains from my posted solution come exactly because the CPU intensive part of the processing -- the spliting and joining of the records -- can run flat out (100% utilisation) on one CPU, whilst the first thread doing the Input is instantly ready to respond to the completion of each read because it isn't having to perform any CPU intensive processing on each record.

push it onto an array (of hashrefs).

The input is a stream; the output is a stream; What on earth do you need an array of hashrefs for?

I suspect that you will be astonished at what just-this does for the program.

No. I can pretty much call it without trying it. It will run 3 to 5 times slower than a simple:

perl -F"\t" -anle"print join chr(9), @F[2,0,5]" in.tsv > out.tsv

That is, instead of taking the OPs 5+ hours to run it will take closer to 24 hours.

Why? You'll never know unless you try it. And you won't do that.

(And even if you get someone else to do it for you, you won't post the results, because it will show your 'advice' to be nothing more than groundless guessing.)


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