magento2: Storing sessions in Redis causes longer loads

Preconditions (*)

1. Magento Community Edition 2.4.1
2. store sessions with redis
3. store cache with redis
4. varnish as fpc

Steps to reproduce (*)

This is tricky, I haven’t found a way to consistently reproduce this issue. But the behaviour seems a lot like what PR #22829 fixes. After receiving feedback from a customer we noticed that some steps in the checkout takes quite a bit of time, when profiling the checkout with Blackfire we noticed that some end points (f/e estimate-shipping-methods) sometimes take over 4s to complete. It looksl like to be coming from the class Magento\Framework\Session\Savehandler\Redis::read() method, which calls Cm\RedisSession\Handler::read. In the latter it uses usleep to wait for locks. Which on occasion takes 2s or more.

session config in env.php 'session' => [ 'save' => 'redis', 'redis' => [ 'host' => 'x.x.x.x', 'port' => 'xxxx', 'password' => '', 'timeout' => '2.5', 'persistent_identifier' => '', 'database' => '1', 'compression_threshold' => '0', 'compression_library' => 'gzip', 'log_level' => '2', 'max_concurrency' => '6', 'break_after_frontend' => '5', 'break_after_adminhtml' => '30', 'first_lifetime' => '600', 'bot_first_lifetime' => '60', 'bot_lifetime' => '7200', 'disable_locking' => '0', 'min_lifetime' => '60', 'max_lifetime' => '2592000' ] ],

Expected result (*)

Less lockout time

Actual result (*)

Lockouts +2s

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Please provide Severity assessment for the Issue as Reporter. This information will help during Confirmation and Issue triage processes.

• Severity: S0 - Affects critical data or functionality and leaves users without workaround.
• Severity: S1 - Affects critical data or functionality and forces users to employ a workaround.
• Severity: S2 - Affects non-critical data or functionality and forces users to employ a workaround.
• Severity: S3 - Affects non-critical data or functionality and does not force users to employ a workaround.
• [] Severity: S4 - Affects aesthetics, professional look and feel, “quality” or “usability”.