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Troubleshoot Azure Cache for Redis latency and timeouts

  • Artikkeli

A client operation that doesn't receive a timely response can result in a high latency or timeout exception. An operation could time out at various stages. Where the timeout comes from helps to determine the cause and the mitigation.

This section discusses troubleshooting for latency and timeout issues that occur when connecting to Azure Cache for Redis.

Note

Several of the troubleshooting steps in this guide include instructions to run Redis commands and monitor various performance metrics. For more information and instructions, see the articles in the Additional information section.

Client-side troubleshooting

Here's the client-side troubleshooting.

Traffic burst and thread pool configuration

Bursts of traffic combined with poor ThreadPool settings can result in delays in processing data already sent by the Redis server but not yet consumed on the client side. Check the metric "Errors" (Type: UnresponsiveClients) to validate if your client hosts can keep up with a sudden spike in traffic.

Monitor how your ThreadPool statistics change over time using an example ThreadPoolLogger. You can use TimeoutException messages from StackExchange.Redis to further investigate:

    System.TimeoutException: Timeout performing EVAL, inst: 8, mgr: Inactive, queue: 0, qu: 0, qs: 0, qc: 0, wr: 0, wq: 0, in: 64221, ar: 0,
    IOCP: (Busy=6,Free=999,Min=2,Max=1000), WORKER: (Busy=7,Free=8184,Min=2,Max=8191)

In the preceding exception, there are several issues that are interesting:

  • Notice that in the IOCP section and the WORKER section you have a Busy value that is greater than the Min value. This difference means your ThreadPool settings need adjusting.
  • You can also see in: 64221. This value indicates that 64,221 bytes were received at the client's kernel socket layer but weren't read by the application. This difference typically means that your application (for example, StackExchange.Redis) isn't reading data from the network as quickly as the server is sending it to you.

You can configure your ThreadPool Settings to make sure that your thread pool scales up quickly under burst scenarios.

Large key value

For information about using multiple keys and smaller values, see Consider more keys and smaller values.

You can use the redis-cli --bigkeys command to check for large keys in your cache. For more information, see redis-cli, the Redis command line interface--Redis.

  • Increase the size of your VM to get higher bandwidth capabilities
    • More bandwidth on your client or server VM might reduce data transfer times for larger responses.
    • Compare your current network usage on both machines to the limits of your current VM size. More bandwidth on only the server or only on the client might not be enough.
  • Increase the number of connection objects your application uses.
    • Use a round-robin approach to make requests over different connection objects

High CPU on client hosts

High client CPU usage indicates the system can't keep up with the work assigned to it. Even though the cache sent the response quickly, the client might fail to process the response in a timely fashion. Our recommendation is to keep client CPU less 80%. Check the metric "Errors" (Type: UnresponsiveClients) to determine if your client hosts can process responses from Redis server in time.

Monitor the client's system-wide CPU usage using metrics available in the Azure portal or through performance counters on the machine. Be careful not to monitor process CPU because a single process can have low CPU usage but the system-wide CPU can be high. Watch for spikes in CPU usage that correspond with timeouts. High CPU might also cause high in: XXX values in TimeoutException error messages as described in the [Traffic burst] section.

Note

StackExchange.Redis 1.1.603 and later includes the local-cpu metric in TimeoutException error messages. Ensure you are using the latest version of the StackExchange.Redis NuGet package. Bugs are regularly fixed in the code to make it more robust to timeouts. Having the latest version is important.

To mitigate a client's high CPU usage:

  • Investigate what is causing CPU spikes.
  • Upgrade your client to a larger VM size with more CPU capacity.

Network bandwidth limitation on client hosts

Depending on the architecture of client machines, they might have limitations on how much network bandwidth they have available. If the client exceeds the available bandwidth by overloading network capacity, then data isn't processed on the client side as quickly as the server is sending it. This situation can lead to timeouts.

Monitor how your Bandwidth usage change over time using an example BandwidthLogger. This code might not run successfully in some environments with restricted permissions (like Azure web sites).

To mitigate, reduce network bandwidth consumption or increase the client VM size to one with more network capacity. For more information, see Large request or response size.

TCP settings for Linux based client applications

Because of optimistic TCP settings in Linux, client applications hosted on Linux could experience connectivity issues. For more information, see TCP settings for Linux-hosted client applications

RedisSessionStateProvider retry timeout

If you're using RedisSessionStateProvider, ensure you set the retry timeout correctly. The retryTimeoutInMilliseconds value should be higher than the operationTimeoutInMilliseconds value. Otherwise, no retries occur. In the following example, retryTimeoutInMilliseconds is set to 3000. For more information, see ASP.NET Session State Provider for Azure Cache for Redis and ASP.NET Output Cache Provider for Azure Cache for Redis.

<add 
    name="AFRedisCacheSessionStateProvider"
    type="Microsoft.Web.Redis.RedisSessionStateProvider"
    host="enbwcache.redis.cache.windows.net"
    port="6380"
    accessKey="..."
    ssl="true"
    databaseId="0"
    applicationName="AFRedisCacheSessionState"
    connectionTimeoutInMilliseconds = "5000"
    operationTimeoutInMilliseconds = "1000"
    retryTimeoutInMilliseconds="3000"
>

Server-side troubleshooting

Here's the server-side troubleshooting.

Server maintenance

Planned or unplanned maintenance can cause disruptions with client connections. The number and type of exceptions depends on the location of the request in the code path, and when the cache closes its connections. For instance, an operation that sends a request but doesn't receive a response when the failover occurs might get a time-out exception. New requests on the closed connection object receive connection exceptions until the reconnection happens successfully.

For more information, check these other sections:

To check whether your Azure Cache for Redis had a failover during when timeouts occurred, check the metric Errors. On the Resource menu of the Azure portal, select Metrics. Then create a new chart measuring the Errors metric, split by ErrorType. Once you create this chart, you see a count for Failover.

For more information on failovers, see Failover and patching for Azure Cache for Redis.

High server load

High server load means the Redis server is unable to keep up with the requests, leading to timeouts. The server might be slow to respond and unable to keep up with request rates.

Monitor metrics such as server load. Watch for spikes in Server Load usage that correspond with timeouts. Create alerts on metrics on server load to be notified early about potential impacts.

There are several changes you can make to mitigate high server load:

  • Investigate what is causing high server load such as long-running commands, noted in this article, because of high memory pressure.
  • Scale out to more shards to distribute load across multiple Redis processes or scale up to a larger cache size with more CPU cores. For more information, see Azure Cache for Redis planning FAQs.
  • If your production workload on a C1 cache is negatively affected by extra latency from some internal defender scan runs, you can reduce the effect by scaling to a higher tier offering with multiple CPU cores, such as C2.

Spikes in server load

On C0 and C1 caches, you might see short spikes in server load not caused by an increase in requests a couple times a day while internal defender scanning is running on the VMs. You see higher latency for requests while internal defender scans happen on these tiers. Caches on the C0 and C1 tiers only have a single core to multitask, dividing the work of serving internal defender scanning and Redis requests.

High memory usage

This section was moved. For more information, see High memory usage.

Long running commands

Some Redis commands are more expensive to execute than others. The Redis commands documentation shows the time complexity of each command. Redis command processing is single-threaded. Any command that takes a long time to run can block all others that come after it.

Review the commands that you're issuing to your Redis server to understand their performance impacts. For instance, the KEYS command is often used without knowing that it's an O(N) operation. You can avoid KEYS by using SCAN to reduce CPU spikes.

Using the SLOWLOG GET command, you can measure expensive commands being executed against the server.

Customers can use a console to run these Redis commands to investigate long running and expensive commands.

  • SLOWLOG is used to read and reset the Redis slow queries log. It can be used to investigate long running commands on client side. The Redis Slow Log is a system to log queries that exceeded a specified execution time. The execution time doesn't include I/O operations like talking with the client, sending the reply, and so forth, but just the time needed to actually execute the command. Customers can measure/log expensive commands being executed against their Redis server using the SLOWLOG command.
  • MONITOR is a debugging command that streams back every command processed by the Redis server. It can help in understanding what is happening to the database. This command is demanding and can negatively affect performance. It can degrade performance.
  • INFO - command returns information and statistics about the server in a format that is simple to parse by computers and easy to read by humans. In this case, the CPU section could be useful to investigate the CPU usage. A server load of 100 (maximum value) signifies that the Redis server was busy all the time and was never idle when processing the requests.

Output sample:

# CPU
used_cpu_sys:530.70
used_cpu_user:445.09
used_cpu_avg_ms_per_sec:0
server_load:0.01
event_wait:1
event_no_wait:1
event_wait_count:10
event_no_wait_count:1
  • CLIENT LIST - returns information and statistics about the client connections server in a mostly human readable format.

Network bandwidth limitation

Different cache sizes have different network bandwidth capacities. If the server exceeds the available bandwidth, then data isn't sent to the client as quickly. Client requests could time out because the server can't push data to the client fast enough.

The "Cache Read" and "Cache Write" metrics can be used to see how much server-side bandwidth is being used. You can view these metrics in the portal. Create alerts on metrics like cache read or cache write to be notified early about potential impacts.

To mitigate situations where network bandwidth usage is close to maximum capacity:

StackExchange.Redis timeout exceptions

For more specific information to address timeouts when using StackExchange.Redis, see Investigating timeout exceptions in StackExchange.Redis.

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