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# Sunday, 14 July 2013

Recently we've seen an article Why mobile web apps are slow.

While title mentions web apps, the the criticism is directed purely to javascript language. The answer presented there is twofold:

  • Raw javascript performance is ~5 times slower than performance of native code solving the same task.

    This accounts for the most modern implementations that exploit JIT. Author does not expect that this proportion will be significatly changed in javascript's favor in the near future.

  • Garbage Collection, which is essential part of javascript, does not work well in constrainted environment of mobile devices.

    Here author quotes many references that show that:

    • for GC to work on peer with non-GC application, it needs to have ~6 - 8 times size of memory than an application needs;
    • at the same time for hardware reasons, mobile devices cannot provide such amount of memory;
    • on the other hand with rise of CPU performance, GC pressure rises even faster.

In the end author, while saying about some attempts to change the state, has no final verdict, whether there can be anything done to remedy the problem.

Having roots in C++, we're GC unbelievers. But you know, who will ask your opinion on that, while there are all those modern languages that try to abstract from memory and implicitly or explicitly assume GC: java, C#, javascript, xslt, xquery, and so on.

There always is a solution to avoid GC completely, like C++ and other (e.g. Microsoft's C++/CX, or Apple's ARC) do. But, assuming you're entered GC world, what can be done with it? How do you make it more predictable, less greedy, and probably more fast?

Our arguments are like this.

How does native code manage object graphs?

Today's solution is reference counting along with weak references to break cycles in graph.

Can be GC based on this?

Yes.

In fact upcoming spec of javascript contains weak references. So, provided a developer accurately defines relations in an object graph, one may immediately achieve the same efficiency as native solution.

If one does not use weak references consistently then object cycles can be created, so there can appear a graph that is not referenced anywhere in a program. This graph can be collected with classical GC that scans object roots.

Classical GC part can be used as a debug mode leak detector, and will collect graph cycles at runtime.

Thus, we claim that a hybrid memory management: reference counting with weak references plus classical GC is possible; it will be equal to a native memory management when weak references are properly used, and it will be close to classical GC without use of weak references.

This solution gives a rule to mitigate GC drawbacks: just use weak references in appropriate points, and you can continue to live in GC world, where GC is only a fallback.

Sunday, 14 July 2013 12:20:29 UTC  #    Comments [0] -
javascript | Thinking aloud
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