Description:
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We're using the memcache extension to handle our session storage. We have the following configuration:

memcache support => enabled
memcache.allow_failover => 1 => 1
memcache.chunk_size => 32768 => 32768
memcache.compress_threshold => 20000 => 20000
memcache.default_port => 11211 => 11211
memcache.hash_function => crc32 => crc32
memcache.hash_strategy => consistent => consistent
memcache.lock_timeout => 15 => 15
memcache.max_failover_attempts => 20 => 20
memcache.protocol => ascii => ascii
memcache.redundancy => 1 => 1
memcache.session_redundancy => 5 => 5

Session Support => enabled
Registered save handlers => files user memcache 
session.auto_start => On => On
session.bug_compat_42 => Off => Off
session.bug_compat_warn => Off => Off
session.cache_expire => 180 => 180
session.cache_limiter => nocache => nocache
session.cookie_httponly => Off => Off
session.cookie_lifetime => 43200 => 43200
session.cookie_path => / => /
session.cookie_secure => Off => Off
session.entropy_file => no value => no value
session.entropy_length => 0 => 0
session.gc_divisor => 1000 => 1000
session.gc_maxlifetime => 43200 => 43200
session.gc_probability => 1 => 1
session.hash_bits_per_character => 5 => 5
session.hash_function => 0 => 0
session.name => PHPSESSID => PHPSESSID
session.referer_check => no value => no value
session.save_handler => memcache => memcache
session.save_path => tcp://xxx:11211?persistent=1&weight=1&timeout=1&retry_interval=15,tcp://xxx:11211?persistent=1&weight=1&timeout=1&retry_interval=15,tcp://xxx:11211?persistent=1&weight=1&timeout=1&retry_interval=15,tcp://xxx:11211?persistent=1&weight=1&timeout=1&retry_interval=15 => tcp://xxx:11211?persistent=1&weight=1&timeout=1&retry_interval=15,tcp://xxx:11211?persistent=1&weight=1&timeout=1&retry_interval=15,tcp://xxx:11211?persistent=1&weight=1&timeout=1&retry_interval=15,tcp://xxx:11211?persistent=1&weight=1&timeout=1&retry_interval=15
session.serialize_handler => php => php
session.use_cookies => On => On
session.use_only_cookies => On => On
session.use_trans_sid => 0 => 0

Apparently randomly, we sometimes see a 15s or 30s delay before the server will return data to the client. 

While debugging, we have tried changing the value of memcache.lock_timeout, and find we can manipulate how long the delay lasts for by fiddling with this. e.g. changing from the 15s to 2s results in delays of 2 or 4s. 

We do not believe this is caused by a single session being accessed multiple times concurrently, as we can reproduce the issue by executing the Linux command curl against the landing page, so no login required and presumably a new session initiated for each connection. 

Expected result:
----------------
Every time we initiate a new connection without a session present, the lock timeout should not be hit and the page should return normally.

Actual result:
--------------
Randomly, new connections hit the lock timeout and return in either 15 or 30s.