Проблема блокировки сессии PHP с MemcacheD

Nginx выбрасывает 502 Bad Gateway на session_start(), вызванный внутри скрипта include php.

Хранилище сеансов PHP обрабатывается MemcacheD

# nginx -v
nginx version: nginx/1.4.6 (Ubuntu)

# php5-fpm -v
PHP 5.5.9-1ubuntu4.14 (fpm-fcgi) (built: Oct 28 2015 01:38:24)
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2014 Zend Technologies
    with Zend OPcache v7.0.3, Copyright (c) 1999-2014, by Zend Technologies

# memcached -h
memcached 1.4.14

# pecl list
Installed packages, channel pecl.php.net:
=========================================
Package   Version State
memcached 2.1.0   stable

# php -c /etc/php5/fpm/php.ini -i | grep session
session
session.auto_start => Off => Off
session.cache_expire => 180 => 180
session.cache_limiter => nocache => nocache
session.cookie_domain => no value => no value
session.cookie_httponly => Off => Off
session.cookie_lifetime => 0 => 0
session.cookie_path => / => /
session.cookie_secure => Off => Off
session.entropy_file => /dev/urandom => /dev/urandom
session.entropy_length => 32 => 32
session.gc_divisor => 1000 => 1000
session.gc_maxlifetime => 1440 => 1440
session.gc_probability => 0 => 0
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 => memcached => memcached
session.save_path => 127.0.0.1:11211 => 127.0.0.1:11211
session.serialize_handler => php => php
session.upload_progress.cleanup => On => On
session.upload_progress.enabled => On => On
session.upload_progress.freq => 1% => 1%
session.upload_progress.min_freq => 1 => 1
session.upload_progress.name => PHP_SESSION_UPLOAD_PROGRESS => PHP_SESSION_UPLOAD_PROGRESS
session.upload_progress.prefix => upload_progress_ => upload_progress_
session.use_cookies => On => On
session.use_only_cookies => On => On
session.use_strict_mode => Off => Off
session.use_trans_sid => 0 => 0

# php -c /etc/php5/fpm/php.ini -i | grep memcached
/etc/php5/cli/conf.d/20-memcached.ini,
memcached
memcached support => enabled
libmemcached version => 1.0.8
memcached.compression_factor => 1.3 => 1.3
memcached.compression_threshold => 2000 => 2000
memcached.compression_type => fastlz => fastlz
memcached.serializer => php => php
memcached.sess_binary => no value => no value
memcached.sess_lock_wait => 150000 => 150000
memcached.sess_locking => 1 => 1
memcached.sess_prefix => memc.sess.key. => memc.sess.key.
Registered save handlers => files user memcached
session.save_handler => memcached => memcached

Во время копания системных вызовов я нашел вероятную причину для Bad Gateway.

Сдерживая процесс php5-fpm, я получаю массу из них.

# strace -p 12927 -ff -tt
Process 12927 attached
11:13:01.205991 restart_syscall(<... resuming interrupted call ...>) = 0
11:13:01.309243 sendto(6, "add memc.sess.key.lock.oqr9vso3a"..., 69, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 69
11:13:01.309411 recvfrom(6, "NOT_STORED\r\n", 8196, MSG_DONTWAIT, NULL, NULL) = 12
11:13:01.309535 nanosleep({0, 150000000}, NULL) = 0
11:13:01.459913 sendto(6, "add memc.sess.key.lock.oqr9vso3a"..., 69, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 69
11:13:01.460049 recvfrom(6, "NOT_STORED\r\n", 8196, MSG_DONTWAIT, NULL, NULL) = 12
11:13:01.460118 nanosleep({0, 150000000}, NULL) = 0
11:13:01.610353 sendto(6, "add memc.sess.key.lock.oqr9vso3a"..., 69, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 69
11:13:01.610480 recvfrom(6, "NOT_STORED\r\n", 8196, MSG_DONTWAIT, NULL, NULL) = 12
11:13:01.610521 nanosleep({0, 150000000}, NULL) = 0
11:13:01.760785 sendto(6, "add memc.sess.key.lock.oqr9vso3a"..., 69, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 69
11:13:01.760944 recvfrom(6, "NOT_STORED\r\n", 8196, MSG_DONTWAIT, NULL, NULL) = 12
11:13:01.761064 nanosleep({0, 150000000}, NULL) = 0
11:13:01.911438 sendto(6, "add memc.sess.key.lock.oqr9vso3a"..., 69, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 69
11:13:01.911575 recvfrom(6, "NOT_STORED\r\n", 8196, MSG_DONTWAIT, NULL, NULL) = 12
11:13:01.911643 nanosleep({0, 150000000}, NULL) = 0
11:13:02.061920 sendto(6, "add memc.sess.key.lock.oqr9vso3a"..., 69, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 69
11:13:02.062088 recvfrom(6, "NOT_STORED\r\n", 8196, MSG_DONTWAIT, NULL, NULL) = 12
11:13:02.062211 nanosleep({0, 150000000}, NULL) = 0
11:13:02.212470 sendto(6, "add memc.sess.key.lock.oqr9vso3a"..., 69, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 69
11:13:02.212611 recvfrom(6, "NOT_STORED\r\n", 8196, MSG_DONTWAIT, NULL, NULL) = 12
11:13:02.212693 nanosleep({0, 150000000}, NULL) = 0
11:13:02.362917 sendto(6, "add memc.sess.key.lock.oqr9vso3a"..., 69, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 69
11:13:02.362999 recvfrom(6, 0x2967068, 8196, 64, 0, 0) = -1 EAGAIN (Resource temporarily unavailable)
11:13:02.363065 poll([{fd=6, events=POLLIN}], 1, 5000) = 1 ([{fd=6, revents=POLLIN}])
11:13:02.363196 recvfrom(6, "NOT_STORED\r\n", 8196, MSG_DONTWAIT, NULL, NULL) = 12
11:13:02.363241 nanosleep({0, 150000000}, NULL) = 0
11:13:02.513457 sendto(6, "add memc.sess.key.lock.oqr9vso3a"..., 69, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 69
11:13:02.513531 recvfrom(6, 0x2967068, 8196, 64, 0, 0) = -1 EAGAIN (Resource temporarily unavailable)
11:13:02.513581 poll([{fd=6, events=POLLIN}], 1, 5000) = 1 ([{fd=6, revents=POLLIN}])
11:13:02.513619 recvfrom(6, "NOT_STORED\r\n", 8196, MSG_DONTWAIT, NULL, NULL) = 12
11:13:02.513651 nanosleep({0, 150000000}, ^CProcess 12927 detached

Который вызывает бесконечный цикл до тех пор, пока nginx терпение не закончится, и это вызовет ошибку 502.

Тот же результат из-за ограничения процесса memcached.

Насколько я понимаю, уже есть сеанс с таким идентификатором, и когда memcached пытается добавить тот же ключ, он возвращает NOT_STORED, что приводит к таймауту...

Какой-нибудь намек, куда я должен копать дальше, чтобы найти решение?

Большое спасибо!