On Tue, Apr 2, 2024 at 3:17 PM Corinna Vinschen via Cygwin <cygwin@cygwin.com> wrote: > > On Apr 2 02:04, Martin Wege via Cygwin wrote: > > Hello, > > > > Is there any document which describes how Cygwin and Win32 file > > prefetch and readahead work, and which sizes are used (e.g. always > > read one full page even if only 16 bytes are requested?)? > > I'm not aware of any docs, but again, keep in mind that Cygwin is a > usersapce DLL. We basically do what Windows does for low-level file > access. > > > Quick /usr/bin/stat /etc/profile returns "IO Block: 65536". Does that > > mean the file's block size is really 64k? Is this info per filesystem, > > or hardcoded in Cygwin? > > Hardcoded in Cygwin since 2017, based on a discussion in terms of > file access performance, especially when using stdio.h functions: > > https://cygwin.com/cgit/newlib-cygwin/commit/?id=7bef7db5ccd9c
OUCH. While I can understand the motivation, FAT32 on multi-GB-devices having 64k block size, and Win32 API on Win95/98/ME/Win7 being optimized to that insane block size, it is absolutely WRONG with today's NTFS and even more so with ReFS. This only works if you stream files, but as soon as you are doing random read/writes the performance is terrible due to cache thrashing. That could explain the many complaints about Cygwin's IO performance. So, what can be done? I'm not a benchmarking guru, so I'd like to propose to add a tunable called EXPERIMENTAL_PREFERRED_IO_BLKSIZE to the CYGWIN env variable (marked as "experimental"), so the benchmarking guys can do performance testing without recompiling everything, get perf results for Cygwin 3.6, and decide what to do for Cygwin 3.7. Thanks, Martin -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
