sentry: use read(2) host syscall to perform read on disk-backed MemoryFiles

[dawei-sdw]

The mf.MapInternal()+safemem.CopySeq() approach used right now incurs a lot of page faults without page population. Page-by-page faults incurs a lot of context switching. This commits uses read(2) host syscall instead, which makes one context switch and faults all the pages that are touched during the read.

[ayushr2]

Thanks for the change! I think this is a complimentary change to 0d52b50. In that commit description, you will see that I attached the FIO benchmark results. Could you also attach FIO read benchmark results?

cc @nixprime

[dawei-sdw]

Thanks for the change! I think this is a complimentary change to 0d52b50. In that commit description, you will see that I attached the FIO benchmark results. Could you also attach FIO read benchmark results?

cc @nixprime

Thank you very much for your comments; it is a very helpful suggestion. I will attach the FIO read benchmark results for reference to commit 0d52b50.

[ayushr2]

I will attach the FIO read benchmark results for reference to commit 0d52b50.

I mean could you attach FIO benchmark results in the description of this PR? So it is part of git commit history once this is merged.

[dawei-sdw]

I will attach the FIO read benchmark results for reference to commit 0d52b50.

I mean could you attach FIO benchmark results in the description of this PR? So it is part of git commit history once this is merged.

Sure, I will attach the FIO benchmark results in the commit log of this PR. I haven't researched the FIO benchmark yet, so I will need to look into it first.