[dev] Large items are reducing the memcache hit rate

[Gonçalo Queirós]

Think the previous message tables got scrambled...

Hi there list.

We started investigating the low hit rate of Memcache (around 50%), and
found out that turning large_items off raised the hit rate to around
96%. The problem is due to the get misses generated when trying to get
the respective _os keys of items that are not large items.
This raise from 50% to 96% only happens because in our case we have less
than 1% of large items (the only ones we found were some sessions).

Having this in mind it seems wrong to always try to get a _os key that
in the majority of times will not exist.

We have thought of several fixes to the problem but they all presented
some problems (more memcache requests, low hit rate if memcache mainly
composed by large objects, larger complexity), but at the end we came to
a solution that seems to solve all these problems.

We have thought to use the Memcache flags to store the amount of pieces
that compose a large item.
In summary when we store an object, we also store the number of pieces
that composes it, and when we retrieve an object we check the flags to
see of how many pieces that item is made of.

Ex with a large object (two pieces):

new key        new flags   current key
object_key     2           object_key
object_key_s1  0           object_key_s1
-              -           object_key_os (with value = 2)

With the new code we only need to store and get two objects on Memcache
versus 3 with the current code.

Ex with a small object:
new key      new flags   current key
object_key   1           object_key

With the new code we make one get and are done. With current code we
make a miss trying to get object_key_os and then a hit getting object_key.

We have made a small script to test the flag usage and everything seems
to work as expected:

$memcache = new Memcache();
$memcache->addServer('127.0.0.1');

$slices = 5;
$memcache->set('a', 'va', $slices << 8);
$flags = 0;
$a = $memcache->get('a', $flags);

echo ($flags >> 8); //Prints 5

If you think this solution is suited for the problem described, we can
implement it in the near future.

-- 
Gonçalo Queirós
Eng. Software
*m.* 913918777

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