Faster commit for fread test

[tdhock]

@MichaelChirico suggested adding a performance test for the improvement in #6925 (comment)
the test cases already existed; I added a Faster commit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (8647d44) to head (f11c022).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7022   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          79       79           
  Lines       14677    14678    +1     
=======================================
+ Hits        14486    14487    +1     
  Misses        191      191           

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[github-actions]

Generated via commit f11c022

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 23 seconds
Installing different package versions	9 minutes and 6 seconds
Running and plotting the test cases	2 minutes and 7 seconds

[tdhock]

I don't see any difference on CI

I do see is locally though (see below) so I guess it is highly system-dependent (disk response time).

Code:

git2r::checkout("~/R/data.table", "fread-file-perf-test-version-Faster")
tinfo=atime::atime_pkg_test_info("~/R/data.table")
tres=eval(tinfo$test.call[["fread(colClasses='Date') improved in #6107"]])
plot(tres)

[MichaelChirico]

maybe a name like FasterFS/FasterDiskIO to convey the circumstances under which this might be faster? WDYT about a targeted benchmark that might draw out the difference a bit more, e.g. "read a sharded file", something like

setwd(tempdir())
dir.create(td<-tempfile())
setwd(td)
for (ii in 1:100) fwrite(iris, ii)
lapply(list.files(), fread)

[tdhock]

interesting idea

[tdhock]

This is the result I get on my system, we can see a constant factor difference.

Code:

git2r::checkout("~/R/data.table", "fread-file-perf-test-version-Faster")
tinfo=atime::atime_pkg_test_info("~/R/data.table")
tres=eval(tinfo$test.call[["fread disk overhead improved in #6925"]])
plot(tres)

[tdhock]

still difficult to see on CI

[MichaelChirico]

Great, this looks more faithful as a test -- scaling the number of fread() calls --> scale the number of file.info() calls. Even if this doesn't catch the specific improvement we were after, having something like this will still be a useful performance regression test.