change if order to improve FileStream perf for cases when both buffers are of the same size

[adamsitnik]

When nothing has been written to the buffer yet (_writePos == 0) and the size of the buffer passed to WriteAsync is the same as internal FileStream buffer, we should omit buffering logic and just call the actual strategy directly.

This is ofc an edge case, but since we already have a benchmark for it and it's relatively small change we should IMO change it.

See the Allocated column:

Method	Toolchain	fileSize	userBufferSize	options	Mean	Ratio	Allocated
WriteAsync	\after\corerun.exe	1048576	4096	None	3,708.7 us	1.00	29,321 B
WriteAsync	\before\corerun.exe	1048576	4096	None	3,719.5 us	1.00	55,969 B
WriteAsync	\after\corerun.exe	1048576	4096	Asynchronous	4,141.6 us	0.98	913 B
WriteAsync	\before\corerun.exe	1048576	4096	Asynchronous	4,238.3 us	1.00	27,561 B
WriteAsync	\after\corerun.exe	104857600	4096	None	212,481.6 us	0.86	2,867,896 B
WriteAsync	\before\corerun.exe	104857600	4096	None	246,750.4 us	1.00	5,124,856 B
WriteAsync	\after\corerun.exe	104857600	4096	Asynchronous	348,312.7 us	0.98	960 B
WriteAsync	\before\corerun.exe	104857600	4096	Asynchronous	355,626.4 us	1.00	2,257,880 B

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Issue Details

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When nothing has been written to the buffer yet (_writePos == 0) and the size of the buffer passed to WriteAsync is the same as internal FileStream buffer, we should omit buffering logic and just call the actual strategy directly.

This is ofc an edge case, but since we already have a benchmark for it and it's relatively small change we should IMO change it.

See the Allocated column:

Method	Toolchain	fileSize	userBufferSize	options	Mean	Ratio	Allocated
WriteAsync	\after\corerun.exe	1048576	4096	None	3,708.7 us	1.00	29,321 B
WriteAsync	\before\corerun.exe	1048576	4096	None	3,719.5 us	1.00	55,969 B
WriteAsync	\after\corerun.exe	1048576	4096	Asynchronous	4,141.6 us	0.98	913 B
WriteAsync	\before\corerun.exe	1048576	4096	Asynchronous	4,238.3 us	1.00	27,561 B
WriteAsync	\after\corerun.exe	104857600	4096	None	212,481.6 us	0.86	2,867,896 B
WriteAsync	\before\corerun.exe	104857600	4096	None	246,750.4 us	1.00	5,124,856 B
WriteAsync	\after\corerun.exe	104857600	4096	Asynchronous	348,312.7 us	0.98	960 B
WriteAsync	\before\corerun.exe	104857600	4096	Asynchronous	355,626.4 us	1.00	2,257,880 B

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

[jeffhandley]

@adamsitnik / @carlossanlop / @Jozkee -- do we want to get this in for Preview 5 with the other FileStream follow-up work?

[adamsitnik]

@carlossanlop @Jozkee I've added a comment that hopefully explains why bypassing the cache is possible in this particular scenario. PTAL

Updated perf numbers:

Method	Toolchain	fileSize	userBufferSize	options	Allocated
WriteAsync	\after\corerun.exe	1048576	4096	Asynchronous	937 B
WriteAsync	\before\corerun.exe	1048576	4096	Asynchronous	27,561 B
WriteAsync	\after\corerun.exe	104857600	4096	None	2,867,896 B
WriteAsync	\before\corerun.exe	104857600	4096	None	5,125,608 B
WriteAsync	\after\corerun.exe	104857600	4096	Asynchronous	2,552 B
WriteAsync	\before\corerun.exe	104857600	4096	Asynchronous	2,259,464 B

[Jozkee]

LGTM, thanks.