linuxdcpp-team team mailing list archive

[cologic <1909861@xxxxxxxxxxxxxxxxxx>]

As far as

"I'm not sure if mapped reading even makes sense in cases where the file
is only being read sequentially through once (maybe things were
different back in the days when the code was written?)."

One possible explanation is that all of those mechanisms appeared no
earlier than 2000 or 2001, and would have taken a while to spread enough
to rely on in the early years of DC++, while mmap() was available since
the earliest widely used versions of Linux.

** Patch added: "Remove local help-file build workaround"
   https://bugs.launchpad.net/dcplusplus/+bug/1909861/+attachment/5448724/+files/rm_readmapped_v2.patch

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https://bugs.launchpad.net/bugs/1909861

Title:
  FileReader is not thread safe on Linux

Status in DC++:
  New

Bug description:
  FileReader::readMapped currently modifies the global SIGBUS handler in
  order to catch read errors:

  https://sourceforge.net/p/dcplusplus/code/ci/default/tree/dcpp/FileReader.cpp#l289

  Since the function can be called concurrently from different threads
  (currently hashing/queue recheck/sfv check in DC++) and each of them
  sets and resets the SIGBUS handler, there's a high risk that the
  application will crash in case of read errors as they aren't being
  handler properly.

  More information about the caveats:
  https://www.sublimetext.com/blog/articles/use-mmap-with-care

  These issues are much more likely to happen with AirDC++ as it uses
  multiple threads for hashing. Read errors caused rather nasty crashes
  with corrupted stack traces for one user, but luckily he was able to
  catch the SIGBUS signal with gdb.

  I didn't even spend time in trying to figure out how to make the
  mapped reads work properly, as based on my testing the basic
  FileReader::readCached function is noticeably faster:

  readMapped: 671 files (21.70 GiB) in 9 directories have been hashed in 4 minutes 21 seconds (84.87 MiB/s)
  readCached: 671 files (21.70 GiB) in 9 directories have been hashed in 3 minutes 58 seconds (93.08 MiB/s)

  FileReader::readMapped is now disabled in AirDC++, as I can't see any
  benefits from using it. The included setjmp.h header is even causing
  issues when using clang for compiling on Linux:
  https://bugs.gentoo.org/731676

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