Use file allocation size where it's beneficial

[adamsitnik]

1. Identify places, where we know the file size up-front. Example: File.WriteAll*. File.Copy etc.
2. Ensure that the given source code has benchmarks coverage in the performance repo (contribute new benchmarks).
3. Change the source code to use the new FileStream ctor that allows for specifying allocationSize and specify the exact value (or estimation as OS is smart enough to shrink the file (when it's closed) if it was too much).
4. Compare before vs after. If there is a visible performance or reliability gain, contribute the change to dotnet/runtime.

Since #51111 has not been merged yet, please use it for measurements.

Tagging subscribers to this area: @carlossanlop
See info in area-owners.md if you want to be subscribed.

Issue Details

---

1. Identify places, where we know the file size up-front. Example: File.WriteAll*. File.Copy etc.
2. Ensure that the given source code has benchmarks coverage in the performance repo (contribute new benchmarks).
3. Change the source code to use the new FileStream ctor that allows for specifying allocationSize and specify the exact value (or estimation as OS is smart enough to shrink the file (when it's closed) if it was too much).
4. Compare before vs after. If there is a visible performance or reliability gain, contribute the change to dotnet/runtime.

Since #51111 has not been merged yet, please use it for measurements.

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

[jozkee]

For places where we receive text instead of bytes, e.g: File.WriteAllText, should we try to determine the size in bytes up-front?

[sakno]

Do we need support this for Memory-Mapped Files? Dup of this comment.

[dmex]

@jozkee

Can you please try benchmarking #51298 again with the preallocation changes but adding FILE_FLAG_SEQUENTIAL_SCAN to kernel32.createfile and FILE_SEQUENTIAL_ONLY to ntdll.createfile?

For *Async methods please try using FILE_FLAG_RANDOM_ACCESS to kernel32.createfile or FILE_RANDOM_ACCESS to ntdll.createfile.

WriteAllBytes is a sequential operation but dotnet doesn't use correct flags in these cases... The cache manager is likely trying to prefetch data from the file and potentially degrading the performance when preallocation was used which is not what you want when using functions like WriteAllBytes which are sequential.

See also:
https://devblogs.microsoft.com/oldnewthing/20120120-00/?p=8493

[GSPP]

Why should async IO use RANDOM_ACCESS specifically? Just curious for the reasoning.

[dmex]

@GSPP

Why should async IO use RANDOM_ACCESS specifically? Just curious for the reasoning.

Just to compare the benchmark results with the other benchmarks.

@jozkee reported a "perf regression of 10%" when using preallocation which was unexpected... I'm thinking some of the changes related to 40359 and 51111 might be missing a call somewhere after the handle was opened and potentially trashing/polluting the cache manager when running the benchmarks and by comparing the results of different flags we could probably determine the prediction model that was being used by the cache manager at the time.