[mvassilev]

A full dump of chr20 took 16.5 s on one thread, against 13.6 s for samtools view from BAM
on one thread and 3.6 s on eight. Decoding the columns and building the text both ran on
the thread that writes; only qualities and names were decoded ahead on other threads.

ramdump -@ N, with no region, starts N threads. An RNTuple reader is not shared between
threads, so each opens the file for itself and formats every N-th block of records (those
of a block of names and qualities, 10,000 records) into a text of its own; the main thread
writes the texts in order, with at most 2N waiting.

• A thread never takes two consecutive blocks, so the name and quality readers decode
  nothing ahead. A first version handed the blocks out as they came, and a thread that got
  two in a row started reading ahead blocks that other threads then took: -@ 2 peaked at
  729 MB.
• The threads open the file without RNTuple's cluster cache, which would have every thread
  decompress the whole cluster of each block it takes.
• With a region, or with -c, -@ is ignored.

NA12878 chr20 at 30x (17.7 million records) on a Xeon E5-2690 v4, pinned to one socket (14
cores), file in the page cache, medians of 5 runs in alternating order:

ramdump -h	wall	CPU	memory
develop, one thread	16.45 s	26.9 s	107 MB
-@ 2	13.74 s	26.9 s	297 MB
-@ 4	7.85 s	30.2 s	379 MB
-@ 8	4.59 s	34.3 s	546 MB

On the same machine samtools view -h -@ 8 takes 3.6 s from BAM (21.6 s CPU, 94 MB) and
7.7 s from CRAM 3.1 (39.4 s CPU, 964 MB). One thread of -@ does all its decoding itself,
so -@ 2 gains less than half; past that the time is about the CPU time over N.

The output equals the one-thread output, byte for byte, on 1, 2, 3, 4, 8 and 500 threads,
with and without FLAG filters. A new test dumps 25,000 records on three threads and compares.
All tests pass. No format change.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.65306% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.90%. Comparing base (29428e1) to head (2b10721).

Files with missing lines	Patch %	Lines
tools/ramdump.cxx	81.39%	9 Missing and 7 partials ⚠️
test/ramdump_test.cxx	91.66%	0 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (82.65%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #124      +/-   ##
===========================================
- Coverage    89.02%   88.90%   -0.13%     
===========================================
  Files           26       26              
  Lines         3408     3506      +98     
  Branches      1155     1198      +43     
===========================================
+ Hits          3034     3117      +83     
- Misses         196      204       +8     
- Partials       178      185       +7     

Flag	Coverage Δ
unittests	88.90% <82.65%> (-0.13%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
test/ramdump_test.cxx	98.42% <91.66%> (-0.71%)	⬇️
tools/ramdump.cxx	85.49% <81.39%> (-0.65%)	⬇️

Files with missing lines	Coverage Δ
test/ramdump_test.cxx	98.42% <91.66%> (-0.71%)	⬇️
tools/ramdump.cxx	85.49% <81.39%> (-0.65%)	⬇️

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