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hash: A waiting list match is a formal revalidation
Not only does the waiting list operate on an objcore, it also passes a reference to requests before they reembark a worker. This way when an objcore is already present during lookup we can attempt a cache hit without holding the objhead lock. Instead of repurposing the req::hash_objhead field into an equivalent req::hash_objcore, the field is actually removed. In order to signal that a request comes back from its waiting list, the life time of the req::waitinglist flag is extended until cnt_lookup() is reentered. This change creates a new first step in the lookup process: - rush match (when coming from the waiting list) - hash lookup (unless coming from the waiting list) - objhead lookup (unless there was a rush hit) If the rush match is a hit, the objhead lock is briefly acquired to release req's reference, to rely solely on the objcore's reference like a normal hit. This shifts the infamous waiting list serialization phenomenon to the vary header match. Knowing that a rushed object is guaranteed to lead to a cache hit allows the rush policy to be applied wholesale, instead of exponentially. Only requests incompatible with the objcore vary header may reenter the waiting list, a scenario no different from the spurious rush wakeups when operating solely on objhead updates. If a cacheable object was inserted in the cache, but already expired, this behavior enables cache hits. This can be common with multi-tier Varnish setups where one Varnish server may serve a graced object to an other, but true of any origin server that may serve stale yet valid responses. The waiting list enables a proper response-wide no-cache behavior from now on, but the built-in VCL prevents it by default. This is also the first step towards implementing no-cache and private support at the header field granularity.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,156 @@ | ||
| varnishtest "successful expired waiting list hit" | ||
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| barrier b1 cond 2 | ||
| barrier b2 cond 2 | ||
| barrier b3 cond 2 | ||
| barrier b4 cond 2 | ||
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| server s1 { | ||
| rxreq | ||
| expect req.http.user-agent == c1 | ||
| expect req.http.bgfetch == false | ||
| barrier b1 sync | ||
| barrier b2 sync | ||
| txresp -hdr "Cache-Control: max-age=60" -hdr "Age: 120" | ||
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| rxreq | ||
| expect req.http.user-agent == c3 | ||
| expect req.http.bgfetch == true | ||
| txresp | ||
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| # The no-cache case only works with a complicit VCL, for now. | ||
| rxreq | ||
| expect req.http.user-agent == c4 | ||
| expect req.http.bgfetch == false | ||
| barrier b3 sync | ||
| barrier b4 sync | ||
| txresp -hdr "Cache-Control: no-cache" | ||
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| rxreq | ||
| expect req.http.user-agent == c6 | ||
| expect req.http.bgfetch == false | ||
| txresp -hdr "Cache-Control: no-cache" | ||
| } -start | ||
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| varnish v1 -cliok "param.set default_grace 1h" | ||
| varnish v1 -cliok "param.set thread_pools 1" | ||
| varnish v1 -cliok "param.set debug +syncvsl,+waitinglist" | ||
| varnish v1 -vcl+backend { | ||
| sub vcl_backend_fetch { | ||
| set bereq.http.bgfetch = bereq.is_bgfetch; | ||
| } | ||
| sub vcl_beresp_stale { | ||
| # We just validated a stale object, do not mark it as | ||
| # uncacheable. The object remains available for grace | ||
| # hits and background fetches. | ||
| return; | ||
| } | ||
| sub vcl_beresp_control { | ||
| if (beresp.http.cache-control == "no-cache") { | ||
| # Keep beresp.uncacheable clear. | ||
| return; | ||
| } | ||
| } | ||
| sub vcl_deliver { | ||
| set resp.http.obj-hits = obj.hits; | ||
| set resp.http.obj-ttl = obj.ttl; | ||
| } | ||
| } -start | ||
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| client c1 { | ||
| txreq -url "/stale-hit" | ||
| rxresp | ||
| expect resp.status == 200 | ||
| expect resp.http.x-varnish == 1001 | ||
| expect resp.http.obj-hits == 0 | ||
| expect resp.http.obj-ttl < 0 | ||
| } -start | ||
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| barrier b1 sync | ||
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| client c2 { | ||
| txreq -url "/stale-hit" | ||
| rxresp | ||
| expect resp.status == 200 | ||
| expect resp.http.x-varnish == "1004 1002" | ||
| expect resp.http.obj-hits == 1 | ||
| expect resp.http.obj-ttl < 0 | ||
| } -start | ||
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| varnish v1 -expect busy_sleep == 1 | ||
| barrier b2 sync | ||
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| client c1 -wait | ||
| client c2 -wait | ||
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| varnish v1 -vsl_catchup | ||
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| varnish v1 -expect cache_miss == 1 | ||
| varnish v1 -expect cache_hit == 1 | ||
| varnish v1 -expect cache_hit_grace == 0 | ||
| varnish v1 -expect s_bgfetch == 0 | ||
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| client c3 { | ||
| txreq -url "/stale-hit" | ||
| rxresp | ||
| expect resp.status == 200 | ||
| expect resp.http.x-varnish == "1006 1002" | ||
| expect resp.http.obj-hits == 2 | ||
| expect resp.http.obj-ttl < 0 | ||
| } -run | ||
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| varnish v1 -vsl_catchup | ||
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| varnish v1 -expect cache_miss == 1 | ||
| varnish v1 -expect cache_hit == 2 | ||
| varnish v1 -expect cache_hit_grace == 1 | ||
| varnish v1 -expect s_bgfetch == 1 | ||
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| # The only way for a plain no-cache to be hit is to have a non-zero keep. | ||
| varnish v1 -cliok "param.set default_ttl 0" | ||
| varnish v1 -cliok "param.set default_grace 0" | ||
| varnish v1 -cliok "param.set default_keep 1h" | ||
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| client c4 { | ||
| txreq -url "/no-cache-hit" | ||
| rxresp | ||
| expect resp.status == 200 | ||
| expect resp.http.x-varnish == 1009 | ||
| expect resp.http.obj-hits == 0 | ||
| expect resp.http.obj-ttl <= 0 | ||
| } -start | ||
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| barrier b3 sync | ||
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| client c5 { | ||
| txreq -url "/no-cache-hit" | ||
| rxresp | ||
| expect resp.status == 200 | ||
| expect resp.http.x-varnish == "1012 1010" | ||
| expect resp.http.obj-hits == 1 | ||
| expect resp.http.obj-ttl <= 0 | ||
| } -start | ||
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| varnish v1 -expect busy_sleep == 2 | ||
| barrier b4 sync | ||
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| client c4 -wait | ||
| client c5 -wait | ||
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| varnish v1 -vsl_catchup | ||
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| varnish v1 -expect cache_miss == 2 | ||
| varnish v1 -expect cache_hit == 3 | ||
| varnish v1 -expect cache_hit_grace == 1 | ||
| varnish v1 -expect s_bgfetch == 1 | ||
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| # No hit when not on the waiting list | ||
| client c6 { | ||
| txreq -url "/no-cache-hit" | ||
| rxresp | ||
| expect resp.status == 200 | ||
| expect resp.http.x-varnish == 1014 | ||
| expect resp.http.obj-hits == 0 | ||
| expect resp.http.obj-ttl <= 0 | ||
| } -run |