[Webkit-unassigned] [Bug 43987] New: Downloading using XHR is much slower than before

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=43987

           Summary: Downloading using XHR is much slower than before
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Other
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: behrooz.noorizadeh at nokia.com


On a simple test case, I tried to download a 5Mb file using XHR request. The download time on the latest WebKit is much more than the time I tried it a few months ago. Looking into traces, it seems most of the time is spent in string concatenation:

WebCore::ScriptString::operator+=(WebCore::String const&)

I think, this issue is introduced since the introduction of the new JS parser. Specifically, JavaScriptCore/wtf/Vector.h has templates for AlignedBuffer with the size of 1 byte to 64 bytes. 64 bytes is too small for downloading big chunks of data. Basically, what is happening is that we keep expanding the vector by only 64 bytes many times and thats why the ScriptString::operator+ is so slow.

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