[Webkit-unassigned] [Bug 207882] New: Inlined NetworkCache should be still mmap-ed

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=207882

            Bug ID: 207882
           Summary: Inlined NetworkCache should be still mmap-ed
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Page Loading
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ysuzuki at apple.com
                CC: beidson at apple.com

If the file size is smaller than the threshold (16KB), we put metadata, header, and body into one file, and disabling mmap feature for this.
Previously, we are using mmap path for 4KB~ data in macOS. This sounds like a memory regression.

Disabling mmap means that we have copy of content in memory in WebProcess, and this content cannot be discarded easily because it is not file-backed.
File-backed mmap-ed content can be potentially discarded efficiently by OS (just purging the content and wiring it to disk).

We should first try 4KB threshold in A/B test to see the effect.
If this is effective,

1. We should consider decreasing this threshold in macOS since page size of macOS is 4KB.
2. We should try mmap-ping inlined body too. mmapping a part of metadata/header too, and using this file handle w/ offset.

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