[EXTERNAL] - Re: Build Options for maximum performance of SHA256

[Michael Wojcik]

> From: openssl-users <openssl-users-bounces at openssl.org> On Behalf Of
> opensslmailing via openssl-users
> Sent: Saturday, 10 February, 2024 13:12
> 
>  - the file is 4 GiByte of size
>  -  I'm  reading  the  file using OpenSSL BIO mechanism with the "rb"
>  option, so read as binary

That is going to be slow. How slow depends on a number of factors, but using stdio for something of that size is definitely sub-optimal.

It would be much better to map it into memory. If your application is 64-bit, you might be able to map the whole thing, though how well Windows and your particular system will cope with that I couldn't say. If your application is 32-bit, then obviously you can't, since your entire address space is only 4GiB (and Windows doesn't let you use all of that). So process it in chunks; even in 32-bit you can probably map 1GiB at a time, I think.

See the CreateFileMapping and MapViewOfFile APIs.

>  -  I  don't  know  how the other program (btw. it is fchash from the
>  FastCopy program) reads the file

Just from the name, I'd guess it uses memory-mapped I/O.

-- 
Michael Wojcik