[llvm-bugs] [Bug 44194] New: Degraded performance after closing and opening a filebuf

via llvm-bugs llvm-bugs at lists.llvm.org
Sun Dec 1 05:37:27 PST 2019


            Bug ID: 44194
           Summary: Degraded performance after closing and opening a
           Product: libc++
           Version: 9.0
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: alex at grundis.de
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

When creating a `std::filebuf` an internal buffer of size 4kB is allocated and
used for reading/writing. On calling `close` on that filebuf that buffer is
deallocated falling back to a small internal buffer (8B IIRC). This is then
used for reading/writing when opening a new file with `std::filebuf::open`.

This small buffer leads to way worse performance than the original 4kB buffer.
I'm considering this a bug, as at least the users expectation would be to that
close and reopen a filebuf would be cheaper than recreating the whole instance.
This matters e.g. when using the filebuf in a loop.

Note: As filebuf is used by [io]fstream, the argument does apply to all
standard fstreams.

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