Jon, do you happen to have unit test code that reproduces this that I
can plug in to an app? I'd like to reproduce this.
On Thu, Mar 4, 2010 at 1:31 PM, Jon Blower <jon.blo...@gmail.com> wrote:
> Hi,
>
> I have a GAE-J application that involves requesting dynamically-
> generated images. The application has fairly high latency (~1s per
> request). I'm stress-testing the application using JMeter, hitting
> GAE with four simultaneous threads, each looping through a fixed set
> of requests. I get a consistent pattern, in which the first twenty or
> thirty requests succeed, then every subsequent request fails. The
> server log shows that the failures occur during a memcache put
> operation:
>
> "com.google.appengine.api.memcache.stdimpl.GCacheException: Policy
> prevented put operation
> at com.google.appengine.api.memcache.stdimpl.GCache.put(GCache.java:
> 165)"
>
> If I stop the stress test for a couple of minutes, then restart it I
> get the same pattern: success for the first 20-30 requests, then this
> failure thereafter. It's not the same requests that fail each time.
> Also, the data I'm storing in the memcache is always 1MB or less. My
> memcache set policy is the default of SET_ALWAYS.
>
> Is there some policy restricting the number of memcache puts I can do
> per second perhaps? Or is there a thread safety issue?
>
> Here are a few more details about my application. I have stored high-
> resolution images in the persistent store by breaking them up into
> chunks of size 1MB or less. I have layered memcache above the
> persistent store. So I have code that looks like this:
>
> public byte[] requestChunk(String chunkId) {
> byte[] chunk = searchMemcache(chunkId);
> if (chunk != null) return chunk;
> chunk = searchPersistentStore(chunkId);
> if (chunk != null) {
> putChunkInMemcache(chunkId, chunk); // *** This is where the
> errors come from! ***
> return chunk;
> }
> return null;
> }
>
> It's the putChunkInMemcache() method that fails during the stress
> test.
>
> Any guidance much appreciated!
> Jon
>
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>
--
Ikai Lan
Developer Programs Engineer, Google App Engine
http://googleappengine.blogspot.com | http://twitter.com/app_engine
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