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In versions of Tarantool before 1.10, the server needs to be restarted to change this parameter. The Tarantool server will be unavailable while restarting from .xlog files, unless you restart it using :ref:`hot standby <index-hot_standby>` mode. In the latter case, nearly 100% server availability is guaranteed.
Clean up the database.
Check the indicators of memory fragmentation:
-- requesting quota_used_ratio value
tarantool> box.slab.info().quota_used_ratio

-- requesting items_used_ratio value
tarantool> box.slab.info().items_used_ratio
In case of heavy memory fragmentation (``quota_used_ratio`` is getting close to 100%, ``items_used_ratio`` is about 50%), we recommend restarting Tarantool in the :ref:`hot standby <index-hot_standby>` mode.
Problem: Tarantool generates too heavy CPU load
The :ref:`transaction processor thread <thread_model>` consumes over 60% CPU.
Attach to the Tarantool instance with :ref:`tt <tt-cli>` utility, analyze the query statistics with :doc:`box.stat() </reference/reference_lua/box_stat>` and spot the CPU consumption leader. The following commands can help:
-- checking the RPS of calling stored procedures
tarantool> box.stat().CALL.rps
The critical RPS value is 75 000, boiling down to 10 000 - 20 000 for a rich Lua application (a Lua module of 200+ lines).
-- checking RPS per query type
tarantool> box.stat().<query_type>.rps
The critical RPS value for SELECT/INSERT/UPDATE/DELETE requests is 100 000.
If the load is mostly generated by SELECT requests, we recommend adding a :ref:`slave server <replication-bootstrap>` and let it process part of the queries.
If the load is mostly generated by INSERT/UPDATE/DELETE requests, we recommend :ref:`sharding the database <sharding>`.
Problem: Query processing times out
All reasons that we discuss here can be identified by messages in Tarantool's log file, all starting with the words ``'Too long...'``.
Both fast and slow queries are processed within a single connection, so the readahead buffer is cluttered with slow queries.
Increase the readahead buffer size (:ref:`box.cfg{readahead} <cfg_networking-readahead>` parameter).
This parameter can be changed on the fly, so you don't need to restart Tarantool. Attach to the Tarantool instance with :ref:`tt <tt-cli>` utility and call ``box.cfg{}`` with a new ``readahead`` value:
-- changing the readahead value
tarantool> box.cfg{readahead = 10 * 1024 * 1024}
**Example:** Given 1000 RPS, 1 Кbyte of query size, and 10 seconds of maximal query processing time, the minimal readahead buffer size must be 10 Mbytes.
On the business logic level, split fast and slow queries processing by different connections.
Slow disks.
Check disk performance (use `iostat <https://linux.die.net/man/1/iostat>`_, `iotop <https://linux.die.net/man/1/iotop>`_ or `strace <https://linux.die.net/man/1/strace>`_ utility to check ``iowait`` parameter) and try to put .xlog files and snapshot files on different physical disks (i.e. use different locations for :ref:`wal_dir <cfg_basic-wal_dir>` and :ref:`memtx_dir <cfg_basic-memtx_dir>`).
Problem: Replication "lag" and "idle" contain negative values
This is about ``box.info.replication.(upstream.)lag`` and ``box.info.replication.(upstream.)idle`` values in :doc:`/reference/reference_lua/box_info/replication` section.
Operating system clock on the hosts is not synchronized, or the NTP server is faulty.
Check NTP server settings.
If you found no problems with the NTP server, just do nothing then. Lag calculation uses operating system clock from two different machines. If they get out of sync, the remote master clock can get consistently behind the local instance’s clock.
Problem: Replication "idle" keeps growing, but no related log messages appear
This is about ``box.info.replication.(upstream.)idle`` value in :doc:`/reference/reference_lua/box_info/replication` section.

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testing_project / Documentation: book/admin/troubleshoot — English

All reasons that we discuss here can be identified by messages in Tarantool's log file, all starting with the words ``'Too long...'``.
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