jvgutierrez opened a new issue, #12177:
URL: https://github.com/apache/trafficserver/issues/12177
while debugging a spike in cache write time
`CacheWriteTime:%<{TS_MILESTONE_CACHE_OPEN_WRITE_END-TS_MILESTONE_CACHE_OPEN_WRITE_BEGIN}msdms>`
we noticed that we are getting some cache write time higher than 0 (most of
them reporting a WL_MISS) even with `Cache-Control: private` responses from the
origin server:
```
Date:2025-04-10 Time:18:55:39 ConnAttempts:0 ConnReuse:0 TTFetchHeaders:290
ClientTTFB:299 CacheReadTime:0 CacheWriteTime:8 TotalSMTime:299
TotalPluginTime:0 ActivePluginTime:0 TotalTime:299
OriginServer:mw-api-ext-ro.discovery.wmnet OriginServerTime:290
CacheResultCode:TCP_MISS CacheWriteResult:WL_MISS ReqMethod:GET RespStatus:200
OriginStatus:200
ReqURL:http://en.wikipedia.org/w/api.php?action=query&format=json&list=random&rnlimit=1&rnnamespace=0
ReqHeader:Host:en.wikipedia.org BerespHeader:Set-Cookie:-
BerespHeader:Cache-Control:private, must-revalidate, max-age=0
BerespHeader:Connection:- RespHeader:X-Cache-Int:cp3073 miss
RespHeader:Backend-Timing:D=45089 t=1744311339316247
Date:2025-04-10 Time:18:57:15 ConnAttempts:0 ConnReuse:70 TTFetchHeaders:672
ClientTTFB:682 CacheReadTime:0 CacheWriteTime:10 TotalSMTime:683
TotalPluginTime:0 ActivePluginTime:0 TotalTime:683
OriginServer:mw-web-ro.discovery.wmnet OriginServerTime:-1
CacheResultCode:TCP_IMS_MISS CacheWriteResult:- ReqMethod:GET RespStatus:304
OriginStatus:200
ReqURL:http://de.wikipedia.org/wiki/Krieg_in_Israel_und_Gaza_seit_2023
BerespHeader:Set-Cookie:- BerespHeader:Cache-Control:private, must-revalidate,
max-age=0 BerespHeader:Connection:- RespHeader:X-Cache-Int:cp3073 miss
RespHeader:Backend-Timing:-
```
A naive attempt of getting rid of those by telling ATS to not store a
response `ts.http.set_server_resp_no_store(1)` in lua if Cache-Control: private
was found didn't have any impact. The question here is why is ATS taking time
(between 8 and 12ms) attempting to get a write lock for responses that are
uncacheable? I'd expect to see a CacheWriteTime of 0 or -1 for every request
where the origin server includes a `Cache-Control: private` header
related config settings:
```
proxy.config.cache.enable_read_while_writer: 0
proxy.config.http.cache.open_write_fail_action: 0
proxy.config.http.cache.max_open_write_retries: 1
proxy.config.cache.mutex_retry_delay: 2
```
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