gets in binary mode is unnecessarily slow

[Forty-Bot]

When using memcached_mget with a single key and cas enabled, the client will send a GETKQ request, wait for it to be acked by the server, and then follows it up by a NOP request. However, Linux delays sending acks until it has some data to reply with or until a timeout occurs. The above request pattern results in the timeout occuring every time the client makes a request. This timeout may be on the order of dozens of milliseconds, increasing the latency of gets commands by up to 1000x that of get commands.

In contrast, memcached_get sends a single GETK request which receives an immediate response, avoiding the delayed ack timeout. However, the CAS value cannot be retrieved with is API.

I have recorded an example PCAP showing the latency difference between memcached_get and memcached_gets in text/binary modes (with the CAS behavior enabled). gets in binary mode takes around 400 ms total, while all other operations take around 1 ms total. This example only performs 10 requests per mode; with 1000s of requests an even larger different may be observed.

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I recommend the following improvements:

• Send a single GETK when key_length is 1 (i.e. as if mget_mode is false).
• Don't flush memcached_io_writev until sending the final NOP (or at least until sending the final GETKQ).
• Add an API to retrieve the CAS value without mget.

[m6w6]

The binary protocol has been deprecated by memcached in v1.6.0
Does enabling the TCP_NODELAY behavior with the binary protocol fix your observed issue?

[Forty-Bot]

TCP_NODELAY will not address the problem, since the delay occurs on the server, not the client.

The binary protocol has been deprecated by memcached in v1.6.0

And yet, libmemcached still does not support the meta protocol, which is the designated replacement for the features provided by the binary protocol.

[m6w6]

Yes, someone should write a modern client library with the meta protocol in mind.
There's so much needless gunk and feature creep in this library which is simply in the way.