/**
*
* Simple CPU throttling implementation.
*
* For this setup, a sandbox attempting to use 100% of cpu over an
* extended period will end up throttled, in the steady-state, to
* 10% of cpu.
*
* Very simple mechanism to begin with. "ctime" is measured for the
* sandbox, being the cumulative time charged to the user (directly or
* indirectly) by the OS for that process. If the average increase in
* ctime over a time period is over 10% (targetRate) of that time period,
* throttling kicks in, and the process will be paused/unpaused via
* signals on a duty cycle.
*
* Left for future work: more careful shaping of CPU throttling, and
* factoring in a team-site level credit system or similar.
*
*/
import * as pidusage from '@gristlabs/pidusage';
import * as log from 'app/server/lib/log';
/**
* Parameters related to throttling.
*/
export interface ThrottleTiming {
dutyCyclePositiveMs: number; // when throttling, how much uninterrupted time to give
// the process before pausing it. The length of the
// non-positive cycle is chosen to achieve the desired
// cpu usage.
samplePeriodMs: number; // how often to sample cpu usage and update throttling
targetAveragingPeriodMs: number; // (rough) time span to average cpu usage over.
minimumAveragingPeriodMs: number; // minimum time span before throttling is considered.
// No throttling will occur before a process has run
// for at least this length of time.
minimumLogPeriodMs: number; // minimum time between log messages about throttling.
targetRate: number; // when throttling, aim for this fraction of cpu usage
// per unit time.
maxThrottle: number; // maximum ratio of negative duty cycle phases to
// positive.
}
/**
* Some parameters that seem reasonable defaults.
*/
const defaultThrottleTiming: ThrottleTiming = {
dutyCyclePositiveMs: 50,
samplePeriodMs: 1000,
targetAveragingPeriodMs: 20000,
minimumAveragingPeriodMs: 6000,
minimumLogPeriodMs: 10000,
targetRate: 0.25,
maxThrottle: 10,
};
/**
* A sample of cpu usage.
*/
interface MeterSample {
time: number; // time at which sample was made (as reported by Date.now())
cpuDuration: number; // accumulated "ctime" measured by pidusage
offDuration: number; // accumulated clock time for which process was paused (approximately)
}
/**
* A throttling implementation for a process. Supply a pid, and it will try to keep that
* process from consuming too much cpu until stop() is called.
*/
export class Throttle {
private _timing: ThrottleTiming; // overall timing parameters
private _meteringInterval: NodeJS.Timeout | undefined; // timer for cpu measurements
private _dutyCycleTimeout: NodeJS.Timeout | undefined; // driver for throttle duty cycle
private _throttleFactor: number = 0; // relative length of paused phase
private _sample: MeterSample | undefined; // latest measurement.
private _anchor: MeterSample | undefined; // sample from past for averaging
private _nextAnchor: MeterSample | undefined; // upcoming replacement for _anchor
private _lastLogTime: number | undefined; // time of last throttle log message
private _offDuration: number = 0; // cumulative time spent paused
private _stopped: boolean = false; // set when stop has been called
/**
* Start monitoring the given process and throttle as needed.
*/
constructor(private readonly _options: {
pid: number,
logMeta: log.ILogMeta,
timing?: ThrottleTiming
}) {
this._timing = this._options.timing || defaultThrottleTiming;
this._meteringInterval = setInterval(() => this._update(), this._timing.samplePeriodMs);
}
/**
* Stop all activity.
*/
public stop() {
this._stopped = true;
this._stopMetering();
this._stopThrottling();
}
/**
* Read the last cpu usage sample made, for test purposes.
*/
public get testStats(): MeterSample|undefined {
return this._sample;
}
/**
* Measure cpu usage and update whether and how much we are throttling the process.
*/
private async _update() {
// Measure cpu usage to date.
let cpuDuration: number;
try {
cpuDuration = (await pidusage(this._options.pid)).ctime;
} catch (e) {
// process may have disappeared.
log.rawDebug(`Throttle measurement error: ${e}`, this._options.logMeta);
return;
}
const now = Date.now();
const current: MeterSample = { time: now, cpuDuration, offDuration: this._offDuration };
this._sample = current;
// Measuring cpu usage was an async operation, so check that we haven't been stopped
// in the meantime. Otherwise we could sneak in and restart a throttle duty cycle.
if (this._stopped) { return; }
// We keep a reference point in the past called the "anchor". Whenever the anchor
// becomes sufficiently old, we replace it with something newer.
if (!this._anchor) { this._anchor = current; }
if (this._nextAnchor && now - this._anchor.time > this._timing.targetAveragingPeriodMs * 2) {
this._anchor = this._nextAnchor;
this._nextAnchor = undefined;
}
// Keep a replacement for the current anchor in mind.
if (!this._nextAnchor && now - this._anchor.time > this._timing.targetAveragingPeriodMs) {
this._nextAnchor = current;
}
// Check if the anchor is sufficiently old for averages to be meaningful enough
// to support throttling.
const dt = current.time - this._anchor.time;
if (dt < this._timing.minimumAveragingPeriodMs) { return; }
// Calculate the average cpu use per second since the anchor.
const rate = (current.cpuDuration - this._anchor.cpuDuration) / dt;
// If that rate is less than our target rate, don't bother throttling.
const targetRate = this._timing.targetRate;
if (rate <= targetRate) {
this._updateThrottle(0);
return;
}
// Calculate how much time the sandbox was paused since the anchor. This is
// approximate, since we don't line up duty cycles with this update function,
// but it should be good enough for throttling purposes.
const off = current.offDuration - this._anchor.offDuration;
// If the sandbox was never allowed to run, wait a bit longer for a duty cycle to complete.
// This should never happen unless time constants are set too tight relative to the
// maximum length of duty cycle.
const on = dt - off;
if (on <= 0) { return; }
// Calculate the average cpu use per second while the sandbox is unpaused.
const rateWithoutThrottling = (current.cpuDuration - this._anchor.cpuDuration) / on;
// Now pick a throttle level such that, if the sandbox continues using cpu
// at rateWithoutThrottling when it is unpaused, the overall rate matches
// the targetRate.
// one duty cycle lasts: quantum * (1 + throttleFactor)
// (positive cycle lasts 1 quantum; non-positive cycle duration is that of
// positive cycle scaled by throttleFactor)
// cpu use for this cycle is: quantum * rateWithoutThrottling
// cpu use per second is therefore: rateWithoutThrottling / (1 + throttleFactor)
// so: throttleFactor = (rateWithoutThrottling / targetRate) - 1
const throttleFactor = rateWithoutThrottling / targetRate - 1;
// Apply the throttle. Place a cap on it so the duty cycle does not get too long.
// This cap means that low targetRates could be unobtainable.
this._updateThrottle(Math.min(throttleFactor, this._timing.maxThrottle));
if (!this._lastLogTime || now - this._lastLogTime > this._timing.minimumLogPeriodMs) {
this._lastLogTime = now;
log.rawDebug('throttle', {...this._options.logMeta,
throttle: Math.round(this._throttleFactor),
throttledRate: Math.round(rate * 100),
rate: Math.round(rateWithoutThrottling * 100)});
}
}
/**
* Start/stop the throttling duty cycle as necessary.
*/
private _updateThrottle(factor: number) {
// For small factors, let the process run continuously.
if (factor < 0.001) {
if (this._dutyCycleTimeout) { this._stopThrottling(); }
this._throttleFactor = 0;
return;
}
// Set the throttle factor to apply and make sure the duty cycle is running.
this._throttleFactor = factor;
if (!this._dutyCycleTimeout) { this._throttle(true); }
}
/**
* Send CONTinue or STOP signal to process.
*/
private _letProcessRun(on: boolean) {
try {
process.kill(this._options.pid, on ? 'SIGCONT' : 'SIGSTOP');
} catch (e) {
// process may have disappeared
log.rawDebug(`Throttle error: ${e}`, this._options.logMeta);
}
}
/**
* Send CONTinue or STOP signal to process, and schedule next step
* in duty cycle.
*/
private _throttle(on: boolean) {
this._letProcessRun(on);
const dt = this._timing.dutyCyclePositiveMs * (on ? 1.0 : this._throttleFactor);
if (!on) { this._offDuration += dt; }
this._dutyCycleTimeout = setTimeout(() => this._throttle(!on), dt);
}
/**
* Make sure measurement of cpu is stopped.
*/
private _stopMetering() {
if (this._meteringInterval) {
clearInterval(this._meteringInterval);
this._meteringInterval = undefined;
}
}
/**
* Make sure duty cycle is stopped and process is left in running state.
*/
private _stopThrottling() {
if (this._dutyCycleTimeout) {
clearTimeout(this._dutyCycleTimeout);
this._dutyCycleTimeout = undefined;
this._letProcessRun(true);
}
}
}