fscache: load directories only once #205
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If multiple threads access a directory that is not yet in the cache, the directory will be loaded by each thread. Only one of the results is added to the cache, all others are leaked. This wastes performance and memory. On cache miss, add a future object to the cache to indicate that the directory is currently being loaded. Subsequent threads register themselves with the future object and wait. When the first thread has loaded the directory, it replaces the future object with the result and notifies waiting threads. Signed-off-by: Karsten Blees <blees@dcon.de>
| * Looks up or creates a cache entry for the specified key. | ||
| */ | ||
| static struct fsentry *fscache_get(struct fsentry *key) | ||
| { | ||
| struct fsentry *fse; | ||
| struct fsentry *fse, *future, *waiter; | ||
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| EnterCriticalSection(&mutex); |
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Hmm. From http://msdn.microsoft.com/en-us/library/windows/desktop/ms682608%28v=vs.85%29.aspx I expected that the second thread to hit the same code path would have to wait until the first one added the entry, and therefore would not add another entry... (sorry if I am stupidly missing anything)
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The actual loading is done outside the critical section (see lines 290/325 below). We don't want to block concurrent readers while doing expensive IO.
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My bad for missing this! Thanks for the education!
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MSysGit - the development behind Git for Windows » git #236 SUCCESS |
fscache: load directories only once
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fscache: load directories only once
If multiple threads access a directory that is not yet in the cache, the
directory will be loaded by each thread. Only one of the results is added
to the cache, all others are leaked. This wastes performance and memory.
On cache miss, add a future object to the cache to indicate that the
directory is currently being loaded. Subsequent threads register themselves
with the future object and wait. When the first thread has loaded the
directory, it replaces the future object with the result and notifies
waiting threads.
Signed-off-by: Karsten Blees blees@dcon.de