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Performance tuning

MongoDB configuration

From a performance point of view, it is recommended to use MongoDB 3.6 with WireTiger, especially in update-intensive scenarios.

In addition, take into account the following information from the official MongoDB documentation, as it may have impact on performance:

  • Check that ulimit settings in your system are ok. MongoDB provides the following recomendations As described in that document, in RHEL/CentOS you have to create a /etc/security/limits.d/99-mongodb-nproc.conf file, in order to set soft/hard process limit to at least 32000 (check details in the cited document).
  • You should also disable Transparent Huge Pages (HTP) to increment the performance as explain in this document.

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Database indexes

Orion Context Broker doesn't create any index in any database collection (with two exceptions, described at the end of this section), to give flexibility to database administrators. Take into account that index usage involves a tradeoff between read efficiency (usage of indexes generally speeds up reads) and write efficiency (the usage of indexes slows down writes) and storage (indexes consume space in database and mapped RAM memory) and it is the administrator (not Orion) who has to decide what to prioritize.

However, in order to help administrators in this task, the following general indexes are recommended:

  • Collection entities
    • {_id.servicePath: 1, _id.id: 1, _id.type: 1} (note that this is a compound index and key order matters in this case)
    • creDate

In addition, depending on your queries, you may need additional indexes.

  • In the case of using attribute filters in queries (e.g. GET /v2/entities?q=A<10), it is recommended to create indexes for them. In particular if you are filtering by a given attribute 'A' then you should create an index {attrs.A.value: 1}. If you are filtering by several attributes in the same query (e.g. GET /v2/entities?q=A<10;B>20 you should combine them all in a compound index {attrs.A.value: 1, attrs.B.value: 1} (key order doesn't matter in this case).

  • In the case of using orderBy queries (i.e. GET /v2/entities?orderBy=A), it is recommended to create indexes for them. In particular, if you are ordering by a given attribute 'A' in ascending order (i.e. orderBy=A) you should create an index {attrs.A.value: 1}. In the case of ordering by a given attribute 'A' in descending order (i.e. orderBy=!A) you should create an index {attrs.A.value: -1}. In the case of using several attributes for ordering (i.e. orderBy=A,!B,C) you should create a compound index taking into account the ordering direction, i.e. {attrs.A.value: 1, attrs.B.value: -1, attrs.C.value: 1} (key order matters in this case)

  • The above rules have a slight modification when the filter is dateCreate or dateModified. In the case of entity creation/modification date (e.g. GET /v2/entities?dateModified<2019-01-01), use creDate and modDate instead of attrs.A.value. In the case of attribute creation/modification date (e.g. GET /v2/entities?mq=A.dateModified<2019-01-01), use attrs.A.creDate and attrs.A.modDate instead of attrs.A.value).

The only indexes that Orion Context Broker actually ensure are the following ones. Both are ensured on Orion startup or when entities are created.

You can find an analysis about the effect of indexes in this document, although it is based on an old Orion version, so it is probably outdated. In addition, please find some useful references about compound indexes and sort results with indexes from the MongoDB official documentation.

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Write concern

Write concern is a parameter for MongoDB write operations. By default, Orion uses "acknowledged" write concern which means that Orion waits until MongoDB confirms it has applied the operation in memory. You can change this behavior with the -writeConcern CLI option. When "unacknowledged" write concern is configured, Orion doesn't wait to get confirmation, so it can execute write-operations much faster.

Note however that there is a tradeoff between performance and reliability. Using "unacknowledged" write concern you get better performance, but the risk to lose information is higher (as Orion doesn't get any confirmation that the write operation was successful).

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Notification modes and performance

Orion can use different notification modes, depending on the value of -notificationMode.

Default mode is 'transient'. In this mode, each time a notification is sent, a new thread is created to deal with the notification. Once the notification is sent and the response is received, the thread with its connection context is destroyed. This is the recommended mode for low load scenarios. In high level cases, it might lead to a thread exhaustion problem.

Permanent mode is similar, except that the connection context is not destroyed at the end. Thus, new notifications associated to the same connection context (i.e. the same destination URL) can reuse the connection and save the HTTP connection time (i.e. TCP handshake, etc.). Of course, this requires that the server (i.e. the component that receives the notification) also maintains the connection open. Note that while in some cases the permanent mode could improve performance (as it saves the time required to create and destroy HTTP connections), in others it may cause notifications associated to the same connection context to have to wait (as only one of them may use the connection at a time). In other words, only one notification thread can use the connection at a time, so if the notification request/response transmission time exceeds the notification inter-triggering time then threads will block. You can detect this situation when the connectionContext value in statistics is abnormally high.

Finally, threadpool mode is based on a queue for notifications and a pool of worker threads that take notifications from the queue and actually send them on the wire, as shown in the figure below. This is the recommended mode for high load scenarios, after a careful tuning on the queue length and the number of workers. A good starting point is to set the number of workers to the number of expected concurrent clients that send updates, and the queue-limit as N times the number of workers (e.g. N equal to 10, although it could be more or less depending on the expected update burst length). The statistics on the notifQueue block may help you to tune.

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Updates flow control mechanism

Orion implements a flow control mechanism that allow to slow-down the updates flow sent by a client. Using flow control, Orion does not respond immediately to update request, but when the notifications triggered by the update has been sent (totally or partially, depending on the flow control configuration). This way, we avoid Orion saturation due to too much accumulated notifications in the notification queue (which, at the end, causes notifications being discarded when the queue is full).

Flow control mechanism is configured using -notifFlowControl which takes three parameters: -notifFlowControl gauge:stepDelay:maxInterval (which meaning and utilization is explained below). It requires threadpool notification mode (i.e. -notificationMode has to be `threadpool).

Flow control is applied to updates that use the flowControl option (for instance, POST /v2/op/update?options=flowControl. In that case, Orion does not respond immediately to the update and applies a flow control mechanism, which works as follows:

  1. A target queue size is calculated. This calculation is based in the following formula: target = q0 + (1 - gauge) * notifSent, where: q0 is the size of the notification queue before starting to process the update, gauge is a value from 0 to 1 defined globally for Orion at startup time and notifSent are the notifications triggered by the update and added to the notification queue. Note these special cases:
    • If gauge is 1 (aggressive flow control), then target = q0. That is, the target is that queue gets the same size it has before starting to process the update. This is the recommended configuration for gauge.
    • If gauge is 0 (permissive flow control), then target = q0 + notifSent. That is, the target is that queue has the same size it has before starting to process the update. If no concurrent updates occur, this means that flow control mechanism has reached the target even before starting.
  2. Flow control is done in several passes. In each pass, the current notification queue (which increases due to concurrent updates and decreases due to threadpool workers sending notifications) is evaluated, so
    • If current notification queue is equal or less than target, then flow control mechanism returns control and the update response is finally sent
    • If current notification queue is greater than target, then the flow control mechanism waits for a while (stepDelay parameter) and does a new pass.
  3. In order not waiting too much (which eventually could cause a timeout connection close by the client) we have the maxInterval parameter. This specifies and absolute waiting time for clients so, if maxInternval time is reached in a given pass, flow control mechanism returns control, no matter if the target was reached or not.

Flow control is especially interesting in these two cases:

  • Batch updates (POST /v2/update), as they may include several individual entities updates, each one involving potentially several notifications.
  • Updates (no matter if batch or regular) on entities with a 1:N subscriptions relationship (N subscriptions on the same entity), as a single update causes several notifications. Especially when N is large.

A detailed example of flow control in operation can be found in this document.

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Payload and message size and performance

Orion Context Broker uses two default limits related with payloads and HTTP message size. In particular:

  • There is a default limit of 1MB for incoming HTTP request payload
  • There is a default limit of 8MB for outgoing HTTP request messages (including HTTP request line, headers and payload), which applies to notifications and forwarded requests

This limit should suffice for most of the use cases and, at the same time, avoids denial of service due to too large requests. You can change these limits using the following CLI flags:

  • -inReqPayloadMaxSize (in bytes) to change the limit in incoming HTTP request payload
  • -outReqMsgMaxSize (in bytes) to change the limit for outgoing HTTP request messages

Decreasing the limits could have a positive impact on performance, but may impose limitations in Context Broker requests. Increasing the limits may have a negative impact in performance, but will allow bigger requests. In general, it is not recommended to change the defaults.

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HTTP server tuning

The REST API that Orion implements is provided by an HTTP server listening on port 1026 by default (this can be overridden by the -port CLI parameter). You can tune its behavior using the following CLI parameters (see details in the corresponding document):

  • connectionMemory. Sets the size of the connection memory buffer (in kB) per connection used internally by the HTTP server library. Default value is 64 kB.

  • maxConnections. Maximum number of simultaneous connections. Default value is 1020, for legacy reasons, while the lower limit is 1 and there is no upper limit (limited by max number of file descriptors of the operating system).

  • reqPoolSize. Size of thread pool for incoming connections. Default value is 0, meaning no thread pool at all, i.e., a new thread is created to manage each new incoming HTTP request and destroyed after its use. Thread pool mode uses internally the epoll() system call, which is more efficient than the one used when no thread pool is used (poll()). Some performance information regarding this can be found in the documentation of the HTTP server library itself.

  • reqTimeout. The inactivity timeout in seconds before a connection is closed. Default value is 0 seconds, which means infinity. This is the recommended behaviour and setting it to a non-infinite timeout could cause Orion to close the connection before completing the request (e.g. a query request involving several CPr forwards can take a long time). This could be considered an "HTTP-unpolite" behaviour from the point of view of the server (Orion) given that, in this case, it should be the client who decides to close the connection. However, this parameter may be used to limit resource consumption at server side (Orion). Use it with care.

Given that thread creation and destruction are costly operations, it is recommend to use -reqPoolSize in high load scenarios. In particular, according to MHD feedback, the pool should be sized with a value equal or close to number of available CPU cores. If you set -reqPoolSize to a value higher than number of CPU cores then you'll most probably experience performance decrease.

The other three parameters (-reqTimeout, -maxConnections and -connectionMemory) usually work well with their default values.

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Orion thread model and its implications

Orion is a multithread process. With default starting parameters and in idle state (i.e. no load), Orion consumes 4 threads:

  • Main thread (the one that starts the broker, then sleeps forever)
  • Subscription cache synchronization thread (if -noCache is used then this thread is not created)
  • Listening thread for the IPv4 server (if -ipv6 is used then this thread is not created)
  • Listening thread for the IPv6 server (if -ipv4 is used then this thread is not created)

In busy state, the number of threads will be higher. With default configuration, Orion creates a new thread for each incoming request and for each outgoing notification. These threads are destroyed once their work finalizes.

The default configuration is fine for low to medium load scenarios. In high load scenarios you may have a large number of simultaneous requests and notifications so the number of threads may reach the per process operating system level. This is known as the thread exhaustion problem and will cause Orion to not work properly, being unable to deal with new incoming request and outgoing notifications. You can detect that situation by two symptoms.

  • First, a number of threads associated to the process very close to the per process operating system limit.

  • Second, error messages like this appearing in the logs:

    Runtime Error (error creating thread: ...)

In order to avoid this problem, Orion supports thread pools. Using thread pools you can statically set the number of threads that the Orion process uses, removing the dynamics of thread creation/destruction the thread exhaustion problem is avoided. In other words, pools make the behavior of Orion more predictable, as a way of guaranteeing that the Orion process doesn't go beyond the per process operating system thread limit.

There are two pools that can be configured independently:

Using both parameters, in any situation (either idle or busy) Orion consumes a fixed number of threads:

  • Main thread (the one that starts the broker, then sleeps forever)
  • Subscription cache synchronization thread (if -noCache is used then this thread is not created)
  • c listening threads for the IPv4 server (if -ipv6 is used then these threads are not created)
  • c listening threads for the IPv6 server (if -ipv4 is used then these threads are not created)
  • n threads corresponding to the workers in the notification thread pool.

Apart from avoiding the thread exhaustion problem, there is a trade-off between using thread pools and not. On the one side, using thread pools is beneficial as it saves thread creation/destruction time. On the other hand, setting thread pools is a way of "capping" throughput. If the thread workers are busy all the time, at the end the queue saturates and you will end up losing ongoing notifications.

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File descriptors sizing

The following inequity ensures the number of file descriptors used by Orion is below the operating system limit:

max fds > 5 * n + max cons + db pool size + extra

where

  • max fds is the per process file descriptors limit, i.e. the output of the ulimit -n command. It can be changed with ulimit -n <new limit>.
  • n, number of threads in the notification thread pool. The factor 5 is due to that each thread can hold up to 5 connections (libcurl pool).
  • max cons is the size of the thread pool for incoming connections, configured with -reqPoolSize CLI parameter. Note that if you don't use this parameter, default is not using any pool for incoming connections. Thus, a burst of incoming connections large enough could exhaust in theory all available file descriptors.
  • db pool size is the size of the DB connection pool, configured with -dbPoolSize CLI parameter, which default value is 10.
  • extra an amount of file descriptors used by log files, listening sockets and file descriptors used by libraries. There isn't any general rule for this value, but one in the range of 100 to 200 must suffice most of the cases.

If the above inequity doesn't hold, you may have file descriptors exhaustion problem and Orion Context Broker will not work properly. In particular, it may happen that Orion is unable to accept new incoming connections and/or send notifications due to lack of file descriptors.

Note that having a large number of client connections at Orion in CLOSE_WAIT status is not a problem. This is part of the libcurl connection cache strategy, in order to save time by reusing connections. From libcurl email discussion about this topic:

The CLOSE_WAIT sockets are probably the ones that libcurl has in its connection cache but that have been "closed" (a FIN was sent) by the server already but not yet by libcurl. They are not there "indefinitely" (and they really can't be) since the connection cache has a limited size so eventually the old connections should get closed.

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Identifying bottlenecks looking at semWait statistics

The semWait section in the statistics operation output includes valuable information that can be used to detect potential bottlenecks.

  • connectionContext. An abnormally high value in this metric may be due to that many notifications want to use the same permanent connection. In that case, stop using permanent notification mode and use transient or threadpool instead (note that the value of this metric is always 0 if permanent notification mode is not used).

  • dbConnectionPool. Orion keeps a DB connection pool (which size is established with -dbPoolSize). An abnormally high value of this metric means that Orion threads wait too much to get a connection from the pool. This could be due to the size of the pool is insufficient (in that case, increase the value of -dbPoolSize) or that there is some other bottleneck with the DB (in that case, review your DB setup and configuration).

  • request. An abnormally high value in this metric means that threads wait too much before entering the internal logic module that processes the request. In that case, consider to use the "none" policy (note that the value of this metric is always 0 if "none" policy is used). Have a look at the section on mutex policy.

Other metrics (timeStat, transaction and subCache) are for internal low-level semaphores. These metrics are mainly for Orion developers, to help to identify bugs in the code. Their values shouldn't be too high.

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Log impact on performance

Logs can have a severe impact on performance. Thus, in high level scenarios, it is recommended to use -logLevel ERROR or WARN. We have found in some situations that the saving between -logLevel WARN and -logLevel INFO can be around 50% in performance.

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Metrics impact on performance

Metrics measurement may have an impact on performance, as system calls and semaphores are involved. You can disable this feature (thus improving performance) using the -disableMetrics CLI parameter.

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Mutex policy impact on performance

Orion supports four different policies (configurable with -reqMutexPolicy):

  • "all", which ensures that not more than one request is being processed by the internal logic module at the same time

  • "read", which ensures that in a given CB node not more than one read request is being processed by the internal logic module at the same time - write requests can execute concurrently.

  • "write", which ensures that in a given CB node not more than one write request is being processed by the internal logic module at the same time, read requests can execute concurrently

  • "none", which allows all the requests to be executed concurrently.

Default value is "all", mainly due to legacy reasons (a leftover of the times in which some race condition issues may occur). However, for the time being, "none" can safely be used, leading to a better performance (as no thread is blocked waiting for others at the internal logic module entry). In fact, in Active-Active Orion configuration, using something different than "none" doesn't provide any advantage (as the mutex policy is local to the Orion process).

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Outgoing HTTP connection timeout

It may happen that a given notification receiver or a context provider (to which a query/update has been forwarded) takes too long to repond to an HTTP request. In some cases, the receiver is not even listening, so a long timeout (the default one established by the operating system) has to pass before the request can be considered a failure and the sending thread unblocks. This may have a significant impact.

In the case of notifications, it causes that the thread (either transients, persistent or in the thread pool) is blocked. In transient or persistent mode, it involves an idle thread inside the process, counting toward the maximum per-process thread limit but doing no effective work (this can be especially severe in the case of persistent mode, as it will block other notifications trying to send to the same URL). In the second case, it means there are workers in the pool that cannot take on new work while waiting.

In the case of queries/updates forwarded to context providers, the effect is that the original client will take a long time to get the answer. In fact, some clients may give up and close the connection.

In this kind of situations, the -httpTimeout CLI parameter may help to control how long Orion should wait for outgoing HTTP connections, overriding the default operating system timeout.

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Subscription cache

Orion implements a context subscription cache in order to speed up notification triggering. In the current version (this may change in the future), context availability subscriptions doesn't use any cache.

The cache synchronization period is controlled by the -subCacheIval (by default it is 60 seconds). Synchronization involves two different tasks:

  • Reading for changes in the context subscription collection in the database and update the local cache based on it. Note that in a multi-CB configuration, one node may modify the context subscription collection, so this is the way other nodes get aware of the modification.

  • Writing some transient information associated to each subscription into the database. This means that even in mono-CB configurations, you should use a -subCacheIval different from 0 (-subCacheIval 0 is allowed, but not recommended).

Note that in multi-CB configurations with load balancing, it may pass some time between (whose upper limit is the cache refresh interval) a given client sends a notification and all CB nodes get aware of it. During this period, only one CB (the one which processed the subscription and have it in its cache) will trigger notifications based on it. Thus, CB implements "eventual consistency" regarding this.

Note also that there is a tradeoff with the cache refresh interval. Short intervals mean that changes regarding context subscriptions are propagated faster between CB nodes (i.e. it has to pass less time to pass from "eventual consistency" to full consistency) but there is more stress on CB and DB. Large intervals mean that changes take more time to propagate, but the stress on CB and DB is lower.

As a final note, you can disable cache completely using the -noCache CLI option, but that is not a recommended configuration.

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Geo-subscription performance considerations

Current support of georel, geometry and coords expression fields in NGSIv2 subscriptions (aka geo-subscriptions) relies on MongoDB geo-query capabilities. While all other conditions associated to subscriptions (e.g. query filter, etc.) are evaluated on a memory image of the updated entity, the ones related with the georel, geometry and coords of a given subscription need a query in the DB.

However, note that the impact on performance shouldn't be too heavy (the operation invoked in MongoDB is count() which is relatively light).

Our future plan is to implement geo-subscription matching in memory (as the rest of the conditions), but this is not a priority at the moment.

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