Time to Safepoint and HotSpot intrinsics

[Hohensee, Paul]

The compiler could estimate the cost of an iteration using some rough pessimistic heuristic and strip mine based on that. Measuring the safepoint time directly is fragile due to inability to find all the hardware platforms that might run the JVM.

Paul

On 11/22/18, 8:07 AM, "hotspot-dev on behalf of Andrew Haley" <hotspot-dev-bounces at openjdk.java.net on behalf of aph at redhat.com> wrote:

    In several places in HotSpot we have intrinsics that can block for a
    long time. The most extreme cases of this are some crypto accelerators
    that are only bounded by the maximum size of a byte array, 2
    gigabytes.  This leads to extended time to safepoint, where every
    thread has to wait for the intrinsic to complete. It can be on the
    order of seconds.
    
    We could simply limit the maximum buffer size of the intrinsic and
    wrap it in a Java loop with (as usual) a safepoint check. This does
    not noticeably slow anything down, but it does solve the TTSP
    problem. This can be important when multi-megabyte messages are being
    encrypted.
    
    The question is, then, what is the maximum tolerable time that an
    intrinsic should block? I suggest that it should be on the order of a
    millisecond, particularly when garbage collectors are trying to drive
    down the safepoint time. This corresponds to AES/GCM encryption of
    about 128k - 256k batches on a decently fast 64-bit processor. Doing
    encryption in batches like this doesn't significantly slow down
    anything. It is, in a sense, like loop strip mining for primitives.
    
    So, I'm proposing two things: firstly we fix the intrinsics that can
    block for extended periods, and we then declare that no new intrinsic
    code will be accepted without the worst case safepoint time being
    measured and found to be acceptable.
    
    -- 
    Andrew Haley
    Java Platform Lead Engineer
    Red Hat UK Ltd. <https://www.redhat.com>
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