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This is an automatically generated e-mail. To reply, visit:
<a href="https://git.reviewboard.kde.org/r/121570/">https://git.reviewboard.kde.org/r/121570/</a>
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<div>Review request for KDevelop.</div>
<div>By Alexander Richardson.</div>
<p style="color: grey;"><i>Updated Dez. 17, 2014, 2:32 vorm.</i></p>
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<b style="color: #575012; font-size: 10pt;">Repository: </b>
kdevplatform
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Description (updated)</h1>
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<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;"><p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">Kill util/spinlock.h, it is actually worse than QMutex in every case</p>
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">QMutex has changed a lot since this code was introduced and now has the
same kind of logic as SpinLock internally, but implements it a lot more
efficiently. Instead of requiring a QMutex::lock() call and a call to
QWaitCondition::wait() (which is two more compare-and-swap instructions)
it directly uses the futex() syscall.</p>
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">There are also a few problems with the spinlock code:</p>
<ul style="padding: 0;text-rendering: inherit;margin: 0 0 0 1em;line-height: inherit;white-space: normal;">
<li style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: normal;">
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">It spins using testAndSetOrdered() instead of testAndSetRelaxed().</p>
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<li style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: normal;">
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">There is also no need to unlock using sequentially consistent order,
storeRelease() is enough.</p>
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<li style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: normal;">
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">Even worse is the fact that it uses compare-and-swap
(QAtomicInt::testAndSet<em style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: normal;">()) for the spinlock, instead of test-and-set
(QAtomicIn::fetchAndStore</em>()) which would be a lot faster.</p>
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<li style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: normal;">
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">For handling locking with timeout Spinlock uses gettimeofday() which is
not availableon e.g. Windows.</p>
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<li style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: normal;">
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">Finally, the class is poorly named, it isn't a spinlock since it
sleeps in every loop iteration instead of continuously attempting to
acquire the lock.</p>
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<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">A class that actually tries spinning on the lock for a few iterations
before taking the slow path that sleeps MIGHT improve the throughput.
However, finding the right count is hard and will decrease performance
if not chosen correctly.</p>
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">The easiest way to gain a bit of performance in the locking code would
be to use QBasicMutex for all non-recursive QMutexes, since that would
allow inlining of the fast path lock() code. But since the class is not
documented as public we should probably not do that.</p>
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">REVIEW: 121570</p></pre>
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Testing </h1>
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<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;"><p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">Builds</p></pre>
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Diffs</b> </h1>
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<li>language/backgroundparser/urlparselock.cpp <span style="color: grey">(d113b66b3802818805f87c5b68b0b6f0c933f1f0)</span></li>
<li>language/duchain/duchain.h <span style="color: grey">(9180cf13e9970b7dd3347ef757b741adab01b733)</span></li>
<li>util/spinlock.cpp <span style="color: grey">(9b4deeee16ba15e1a9515189a441bc9bd4c1e30f)</span></li>
<li>util/spinlock.h <span style="color: grey">(cbdb904e39679f136348cb5d6aa8af064c811baf)</span></li>
<li>serialization/referencecounting.h <span style="color: grey">(3aa0244377f5cbcb128575585beefa98d7f41626)</span></li>
<li>serialization/referencecounting.cpp <span style="color: grey">(6091c4003fea3573cef779b08b1d4d187c779a7d)</span></li>
<li>util/CMakeLists.txt <span style="color: grey">(a2287bd2ae10d5e7fa64820e46aa514fa6d18cfe)</span></li>
<li>language/duchain/duchain.cpp <span style="color: grey">(7ed779d37bcc7075011a901ab09c24522fb651d4)</span></li>
</ul>
<p><a href="https://git.reviewboard.kde.org/r/121570/diff/" style="margin-left: 3em;">View Diff</a></p>
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