The uWSGI cheaper subsystem – adaptive process spawning

uWSGI provides the ability to dynamically scale the number of running workers via pluggable algorithms. Use uwsgi --cheaper-algos-list to get the list of available algorithms.

Usage

To enable cheaper mode add the cheaper = N option to the uWSGI configuration file, where N is the minimum number of workers uWSGI can run. The cheaper value must be lower than the maximum number of configured workers (workers or processes option).

# set cheaper algorithm to use, if not set default will be used
cheaper-algo = spare

# minimum number of workers to keep at all times
cheaper = 2

# number of workers to spawn at startup
cheaper-initial = 5

# maximum number of workers that can be spawned
workers = 10

# how many workers should be spawned at a time
cheaper-step = 1

This configuration will tell uWSGI to run up to 10 workers under load. If the app is idle uWSGI will stop workers but it will always leave at least 2 of them running. With cheaper-initial you can control how many workers should be spawned at startup. If your average load requires more than minimum number of workers you can have them spawned right away and then “cheaped” (killed off) if load is low enough. When the cheaper algorithm decides that it needs more workers it will spawn cheaper-step of them. This is useful if you have a high maximum number of workers – in the event of a sudden load spike it would otherwise take a lot of time to spawn enough workers one by one.

Setting memory limits

Starting with 1.9.16 rss memory limits can be set to stop cheaper spawning new workers if process count limit was not reached, but total sum of rss memory used by all workers reached given limit.

# soft limit will prevent cheaper from spawning new workers
# if workers total rss memory is equal or higher
# we use 128MB soft limit below (values are in bytes)
cheaper-rss-limit-soft = 134217728

# hard limit will force cheaper to cheap single worker
# if workers total rss memory is equal or higher
# we use 160MB hard limit below (values are in bytes)
cheaper-rss-limit-hard = 167772160

Notes:

  • Hard limit is optional, soft limit alone can be used.
  • Hard value must be higher then soft value, both values shouldn’t be too close to each other.
  • Hard value should be soft + at least average worker memory usage for given app.
  • Soft value is the limiter for cheaper, it won’t spawn more workers, but already running workers memory usage might grow, to handle that reload-on-rss can be set too. To set unbreakable barrier for app memory usage cgroups are recommended.

spare cheaper algorithm

This is the default algorithm. If all workers are busy for cheaper_overload seconds then uWSGI will spawn new workers. When the load is gone it will begin stopping processes one at a time.

spare2 cheaper algorithm

This algorithm is similar to spare, but suitable for large scale by increase workers faster and decrease workers slower.

When number of idle workers is smaller than number specified by cheaper, it spawns (cheaper - number of idle workers) workers. Maximum workers spawned at once can be limited by cheaper-step. For example, cheaper is 4, there are 2 idle workers and cheaper-step is 1, it spawns 1 worker.

When number of idle workers is larger than cheaper, it increments internal counter. When number of idle workers is smaller than or equal to cheaper, reset the counter. When the counter is equal to cheaper-idle, cheap one worker and reset the counter.

Sample configuration:

workers = 64          # maximum number of workers

cheaper-algo = spare2
cheaper = 8           # tries to keep 8 idle workers
cheaper-initial = 8   # starts with minimal workers
cheaper-step = 4      # spawn at most 4 workers at once
cheaper-idle = 60     # cheap one worker per minute while idle

backlog cheaper algorithm

Note

backlog is only available on Linux and only on TCP sockets (not UNIX domain sockets).

If the socket’s listen queue has more than cheaper_overload requests waiting to be processed, uWSGI will spawn new workers. If the backlog is lower it will begin killing processes one at a time.

busyness cheaper algorithm

Note

This algorithm is optional, it is only available if the cheaper_busyness plugin is compiled and loaded.

This plugin implements an algorithm which adds or removes workers based on average utilization for a given time period. It’s goal is to keep more workers than the minimum needed available at any given time, so the app will always have capacity for new requests. If you want to run only minimum number of workers then use the spare or backlog algorithms.

This plugin primarily is used because the way spare and backlog plugins work causes very aggressive scaling behavior. If you set a low cheaper value (for example 1), then uWSGI will keep only 1 worker running and spawn new workers only when that running worker is overloaded. If an app requires more workers, then uWSGI will be spawning and stopping workers all the time. Only during times of very low load the would the minimum number of workers be enough.

The Busyness algorithm tries to do the opposite: spawn as many workers as needed and stop some of them only when there is a good chance that they are not needed. This should lead to a more stable worker count and much less respawns. Since for most of the time we have more worker capacity than actually needed, average application response times should be lower than with other plugins.

Options:

cheaper-overload

Specifies the window, in seconds, for tracking the average busyness of workers. Example:

cheaper-overload = 30

This option will check busyness every 30 seconds. If during the last 30 seconds all workers were running for 3 seconds and idle for the remaining 27 seconds the calculated busyness will be 10% (3/30). This value will decide how fast uWSGI can respond to load spikes. New workers will be spawned at most every cheaper-overload seconds (unless you are running uWSGI on Linux – see cheaper-busyness-backlog-alert for details).

If you want to react to load spikes faster, keep this value low so busyness is calculated more often. Keep in mind this may cause workers to be started/stopped more often than required since every minor spike may spawn new workers. With a high cheaper-overload value the worker count will change much less since longer cycles will eat all short spikes of load and extreme values. Default is 3, for busyness plugin it’s best to use higher value (10-30).

cheaper-step

How many workers to spawn when the algorithm decides they are needed. Default is 1.

cheaper-initial

The number of workers to be started when starting the application. After the app is started the algorithm can stop or start workers if needed.

cheaper-busyness-max

This is the maximum busyness we allow. Every time the calculated busyness for last cheaper-overload seconds is higher than this value, uWSGI will spawn cheaper-step new workers. Default is 50.

cheaper-busyness-min

This is minimum busyness. If current busyness is below this value, the app is considered as being in an “idle cycl” and uWSGI will start counting them. Once we reach needed number of idle cycles uWSGI will kill one worker. Default is 25.

cheaper-busyness-multiplier

This option tells uWSGI how many idle cycles we need before stopping a worker. After reaching this limit uWSGI will stop a worker and reset this counter.

For example:

cheaper-overload = 10
cheaper-busyness-multiplier = 20
cheaper-busyness-min = 25

If average worker busyness is under 25% for 20 checks in a row, executed every 10 seconds (total of 200 seconds), one worker will be stopped. The idle cycles counter will be reset if average busyness jumps above cheaper-busyness-max and we spawn new workers. If during idle cycle counting the average busyness jumps above cheaper-busyness-min but still below cheaper-busyness-max, then the idle cycles counter is adjusted and we need to wait extra one idle cycle. If during idle cycle counting the average busyness jumps above cheaper-busyness-min but still below cheaper-busyness-max three times in a row, then the idle cycle counter is reset.

cheaper-busyness-penalty

uWSGI will automatically tune the number of idle cycles needed to stop worker when worker is stopped due to enough idle cycles and then spawned back to fast (less than the same time we need to cheap worker), then we will increment the cheaper-busyness-multiplier value this value. Default is 1.

Example:

cheaper-overload = 10
cheaper-busyness-multiplier = 20
cheaper-busyness-min = 25
cheaper-busyness-penalty = 2

If average worker busyness is under 25% for 20 checks in a row, executed every 10 seconds (total 200 seconds), one worker will be stopped. If new worker is spawned in less than 200 seconds (counting from the time when we spawned the last worker before it), the cheaper-busyness-multiplier value will be incremented up to 22 (20+2). Now we will need to wait 220 seconds (22*10) to cheap another worker. This option is used to prevent workers from being started and stopped all the time since once we stop one worker, busyness might jump up enough to hit cheaper-busyness-max. Without this, or if tuned poorly, we can get into a stop/start feedback loop .

cheaper-busyness-verbose

This option enables debug logs from the cheaper_busyness plugin.

cheaper-busyness-backlog-alert

This option is only available on Linux. It is used to allow quick response to load spikes even with high cheaper-overload values. On every uWSGI master cycle (default 1 second) the current listen queue is checked. If it is higher than this value, an emergency worker is spawned. When using this option it is safe to use high cheaper-overload values to have smoother scaling of worker count. Default is 33.

cheaper-busyness-backlog-multiplier

This option is only available on Linux. It works just like cheaper-busyness-multiplier, except it is used only for emergency workers spawned when listen queue was higher than cheaper-busyness-backlog-alert.

Emergency workers are spawned in case of big load spike to prevent currently running workers from being overloaded. Sometimes load spike are random and short which can spawn a lot of emergency workers. In such cases we would need to wait several cycles before reaping those workers. This provides an alternate multiplier to reap these processes faster. Default is 3.

cheaper-busyness-backlog-step

This option is only available on Linux. It sets the number of emergency workers spawned when listen queue is higher than cheaper-busyness-backlog-alert. Defaults to 1.

cheaper-busyness-backlog-nonzero

This option is only available on Linux. It will spawn new emergency workers if the request listen queue is > 0 for more than N seconds. It is used to protect the server from the corner case where there is only a single worker running and the worker is handling a long running request. If uWSGI receives new requests they would stay in the request queue until that long running request is completed. With this option we can detect such a condition and spawn new worker to prevent queued requests from being timed out. Default is 60.

Notes regarding Busyness

  • Experiment with settings, there is no one golden rule of what values should be used for everyone. Test and pick values that are best for you. Monitoring uWSGI stats (via Carbon, for instance) will make it easy to decide on good values.

  • Don’t expect busyness to be constant. it will change frequently. In the end, real users interact with your apps in very random way. It’s recommended to use longer –cheaper-overload values (>=30) to have less spikes.

  • If you want to run some benchmarks with this plugin, you should use tools that add randomness to the work load

  • With a low number of workers (2-3) starting new worker or stopping one might affect busyness a lot, if You have 2 workers with busyness of 50%, than stopping one of them will increase busyness to 100%. Keep that in mind when picking min and max levels, with only few workers running most of the time max should be more than double of min, otherwise every time one worker is stopped it might increase busyness to above max level.

  • With a low number of workers (1-4) and default settings expect this plugin will keep average busyness below the minimum level; adjust levels to compensate for this.

  • With a higher number of workers required to handle load, worker count should stabilize somewhere near minimum busyness level, jumping a little bit around this value

  • When experimenting with this plugin it is advised to enable --cheaper-busyness-verbose to get an idea of what it is doing. An example log follows.

    # These messages are logged at startup to show current settings
    [busyness] settings: min=20%, max=60%, overload=20, multiplier=15, respawn penalty=3
    [busyness] backlog alert is set to 33 request(s)
    
    # With --cheaper-busyness-verbose enabled You can monitor calculated busyness
    [busyness] worker nr 1 20s average busyness is at 11%
    [busyness] worker nr 2 20s average busyness is at 11%
    [busyness] worker nr 3 20s average busyness is at 20%
    [busyness] 20s average busyness of 3 worker(s) is at 14%
    
    # Average busyness is under 20%, we start counting idle cycles
    # we have overload=20 and multiplier=15 so we need to wait 300 seconds before we can stop worker
    # cycle we just had was counted as idle so we need to wait another 280 seconds
    # 1 missing second below is just from rounding, master cycle is every 1 second but it also takes some time, this is normal
    [busyness] need to wait 279 more second(s) to cheap worker
    
    # We waited long enough and we can stop one worker
    [busyness] worker nr 1 20s average busyness is at 6%
    [busyness] worker nr 2 20s average busyness is at 22%
    [busyness] worker nr 3 20s average busyness is at 19%
    [busyness] 20s average busyness of 3 worker(s) is at 15%
    [busyness] 20s average busyness is at 15%, cheap one of 3 running workers
    
    # After stopping one worker average busyness is now higher, which is no surprise
    [busyness] worker nr 2 20s average busyness is at 36%
    [busyness] worker nr 3 20s average busyness is at 24%
    [busyness] 20s average busyness of 2 worker(s) is at 30%
    # 30% is above our minimum (20%), but it's still far from our maximum (60%)
    # since this is not idle cycle uWSGI will ignore it when counting when to stop worker
    [busyness] 20s average busyness is at 30%, 1 non-idle cycle(s), adjusting cheaper timer
    
    # After a while our average busyness is still low enough, so we stop another worker
    [busyness] 20s average busyness is at 3%, cheap one of 2 running workers
    
    # With only one worker running we won't see per worker busyness since it's the same as total average
    [busyness] 20s average busyness of 1 worker(s) is at 16%
    [busyness] 20s average busyness of 1 worker(s) is at 17%
    
    # Shortly after stopping second worker and with only one running we have load spike that is enough to hit our maximum level
    # this was just few cycles after stopping worker so uWSGI will increase multiplier
    # now we need to wait extra 3 cycles before stopping worker
    [busyness] worker(s) respawned to fast, increasing cheaper multiplier to 18 (+3)
    
    # Initially we needed to wait only 300 seconds, now we need to have 360 subsequent seconds when workers busyness is below minimum level
    # 10*20 + 3*20 = 360
    [busyness] worker nr 1 20s average busyness is at 9%
    [busyness] worker nr 2 20s average busyness is at 17%
    [busyness] worker nr 3 20s average busyness is at 17%
    [busyness] worker nr 4 20s average busyness is at 21%
    [busyness] 20s average busyness of 4 worker(s) is at 16%
    [busyness] need to wait 339 more second(s) to cheap worker