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Lock-free rate-limit algorithm + bug-fix (#2694)
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@ -19,7 +19,6 @@ package cmd
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import (
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import (
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"errors"
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"errors"
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"net/http"
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"net/http"
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"sync"
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)
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)
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var errTooManyRequests = errors.New("Too many clients in the waiting list")
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var errTooManyRequests = errors.New("Too many clients in the waiting list")
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@ -28,7 +27,6 @@ var errTooManyRequests = errors.New("Too many clients in the waiting list")
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// limit the number of concurrent http requests.
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// limit the number of concurrent http requests.
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type rateLimit struct {
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type rateLimit struct {
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handler http.Handler
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handler http.Handler
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lock sync.Mutex
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workQueue chan struct{}
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workQueue chan struct{}
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waitQueue chan struct{}
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waitQueue chan struct{}
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}
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}
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@ -37,26 +35,26 @@ type rateLimit struct {
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// channel this is in-turn used to rate limit incoming connections in
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// channel this is in-turn used to rate limit incoming connections in
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// ServeHTTP() http.Handler method.
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// ServeHTTP() http.Handler method.
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func (c *rateLimit) acquire() error {
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func (c *rateLimit) acquire() error {
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//lock access to enter waitQueue
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//attempt to enter the waitQueue. If no slot is immediately
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c.lock.Lock()
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//available return error.
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// Kick out clients when it is really crowded
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select {
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if len(c.waitQueue) == cap(c.waitQueue) {
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case c.waitQueue <- struct{}{}:
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defer c.lock.Unlock() //unlock after return
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//entered wait queue
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break
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default:
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//no slot available for waiting
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return errTooManyRequests
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return errTooManyRequests
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}
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}
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// Add new element in waitQueue to keep track of clients
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//block attempting to enter the workQueue. If the workQueue is
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// wanting to process their requests
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//full, there can be at most cap(waitQueue) == 4*globalMaxConn
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c.waitQueue <- struct{}{}
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//goroutines waiting here because of the select above.
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select {
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// Unlock now. If we unlock before sending to the waitQueue
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case c.workQueue <- struct{}{}:
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// channel, we can have multiple go-routines blocked on
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//entered workQueue - so remove one waiter. This step
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// sending to the waitQueue (and exceeding the max. number of
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//does not block as the waitQueue cannot be empty.
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// waiting connections.)
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<-c.waitQueue
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c.lock.Unlock()
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}
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// Block to put a waiting go-routine into processing mode.
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c.workQueue <- <-c.waitQueue
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return nil
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return nil
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}
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}
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