avoid ReadFile syscall if we know that there is nothing to read from file

[adamsitnik]

This can save us a single ReadFile sys-call for cases when we know that there is nothing to read from the file, which should happen once for every loop like this:

while (await fileStream.ReadAsync(userBuffer, cancellationToken) != 0)

The perf gain depends on the file size and the number of actual reads. For a small file where we read entire content with first call, we can save up to 10%.

[ghost]

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Issue Details

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This can save us a single ReadFile sys-call for cases when we know that there is nothing to read from the file, which should happen once for every loop like this:

while (await fileStream.ReadAsync(userBuffer, cancellationToken) != 0)

The perf gain depends on the file size and the number of actual reads. For a small file where we read entire content with first call, we can save up to 10%.

Author:	adamsitnik
Assignees:	-
Labels:	area-System.IO
Milestone:	6.0.0