Cygwin&Win32 file prefetch, block sizes?

[Mark Geisert]

On 4/2/2024 3:35 PM, Martin Wege via Cygwin wrote:
> 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.

No comment.

> 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.

That kind of experiment is what folks who can build their own 
cygwin1.dll might do.  I doubt we'd want to make a run-time global disk 
I/O strategy changer available like this, even temporarily.

What could make sense is enhancing Cygwin's posix_fadvise() to support 
POSIX_FADV_RANDOM getting mapped to Windows' FILE_RANDOM_ACCESS flag.
Something like this is currently done for POSIX_FADV_SEQUENTIAL -> 
FILE_SEQUENTIAL_ONLY.  These are per-filedescriptor adjustments and due 
to Windows limitations would apply to a whole file rather than having 
the POSIX behavior of being settable for a byte range within a file.

SHTDI, PTC, and all that :-).

..mark