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services: 'fold-services' memoizes service values.

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Previously 'fold-services' could end up traversing the same services in
the graph several times, which is what this change addresses.

The hit rate on the 'add-data-to-store' cache goves from 9% to 8% on
"guix system build desktop.tmpl -nd", and the number of lookups in that
cache goes from 4458 to 4383.

* gnu/services.scm (fold-services): Turn 'loop' into a monadic procedure
in %STATE-MONAD and use it to memoize values of visited services.
This commit is contained in:
Ludovic Courtès 2019-11-09 21:32:41 +01:00
parent f0034427f5
commit 2a4309de43
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GPG Key ID: 090B11993D9AEBB5
1 changed files with 28 additions and 12 deletions
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40
gnu/services.scm
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@ -775,18 +775,34 @@ TARGET-TYPE; return the root service adjusted accordingly."
(eq? (service-kind service) target-type))
services)
((sink)
(let loop ((sink sink))
(let* ((dependents (map loop (dependents sink)))
(extensions (map (apply-extension sink) dependents))
(extend (service-type-extend (service-kind sink)))
(compose (service-type-compose (service-kind sink)))
(params (service-value sink)))
;; We distinguish COMPOSE and EXTEND because PARAMS typically has a
;; different type than the elements of EXTENSIONS.
(if extend
(service (service-kind sink)
(extend params (compose extensions)))
sink))))
;; Use the state monad to keep track of already-visited services in the
;; graph and to memoize their value once folded.
(run-with-state
(let loop ((sink sink))
(mlet %state-monad ((visited (current-state)))
(match (vhash-assq sink visited)
(#f
(mlet* %state-monad
((dependents (mapm %state-monad loop (dependents sink)))
(visited (current-state))
(extensions -> (map (apply-extension sink) dependents))
(extend -> (service-type-extend (service-kind sink)))
(compose -> (service-type-compose (service-kind sink)))
(params -> (service-value sink))
(service
->
;; Distinguish COMPOSE and EXTEND because PARAMS typically
;; has a different type than the elements of EXTENSIONS.
(if extend
(service (service-kind sink)
(extend params (compose extensions)))
sink)))
(mbegin %state-monad
(set-current-state (vhash-consq sink service visited))
(return service))))
((_ . service) ;SINK was already visited
(return service)))))
vlist-null))
(()
(raise
(condition (&missing-target-service-error