[Webkit-unassigned] [Bug 203423] New: VeryHigh priority loads are actually loading at VeryLow priority

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=203423

            Bug ID: 203423
           Summary: VeryHigh priority loads are actually loading at
                    VeryLow priority
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebKit2
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: nham at apple.com

We tag our resources with one out of 5 priority levels, from ResourceLoadPriority::VeryLow to ResourceLoadPriority::VeryHigh. On Mac and iOS, we translate these levels into one of five CFURLRequestPriority priority levels from 0 (VeryLow) to 4 (VeryHigh) inclusive. CFNetwork then uses these priorities to prioritize which requests should go out first on a connection if there are multiple queued requests. This effect is most pronounced for HTTP/1.1 where it is common to saturate the six-connections-per-host rule, and there are often multiple requests queued up waiting for a connection to free up.

The problem is that we actually tell CFNetwork that we only want 4 priority levels in various places. For instance, in NetworkProcessCocoa::initializeNetworkSettings:

  // toPlatformRequestPriority(WebCore::ResourceLoadPriority::Highest) = 4, so this only gives us 4 priorities not 5!
  _CFNetworkHTTPConnectionCacheSetLimit(kHTTPPriorityNumLevels, toPlatformRequestPriority(WebCore::ResourceLoadPriority::Highest)); 

When CFNetwork detects an out-of-bounds priority level, it resets the priority of the request down to 0 (VeryLow). So this means our VeryHigh priority requests are actually being treated as VeryLow priority requests.

For WebKit2, there is actually a second issue: even if we didn't have this off-by-one error in the call to _CFNetworkHTTPConnectionCacheSetLimit, we'd still be in trouble because CFNetwork doesn't actually apply the connection cache limits from _CFNetworkHTTPConnectionCacheSetLimit to default or ephemeral NSURLSessionConfigurations, which is what we use in NetworkProcess (<rdar://problem/56621205>).

Note that this bug is about CFURLRequestPriority, not NSURLSessionTaskPriority. The latter doesn't do anything for HTTP/1.1 is mainly used to set the stream weight/priority field for HTTP2 connections.

-- 
You are receiving this mail because:
You are the assignee for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.webkit.org/pipermail/webkit-unassigned/attachments/20191025/b94b393c/attachment-0001.htm>