Bug 717761 [1] boils down to the fact that in some cases we are blocking the main thread on I/O that is happening on the cache thread, by having the main thread wait to acquire a lock held by the cache thread.
Apparently, we still have a few public functions exposed from the cache that are synchronous. Bug 695399 [2] is about removing those functions. I believe that we have to get rid of all these synchronous APIs in order to handle bug 660749 [3], because AFAICT, any solution for bug 660749 will potentially require any cache lookup (for HTTPS pages) to require a certificate validation which may require network I/O (for OCSP) and/or disk I/O--even if the cached resource is in the *memory* cache. Currently, we apparently make some assumptions that anything in the memory cache can be safely accessed synchronously, but that would no longer be true--especially since we don't have a solid plan for using the memory cache for anything other than no-store entries, which is something that isn't worth optimizing for. (Having said that, we definitely haven't decided we're only going to use the memory cache for no-store entries. However, bug 665707 makes me think that perhaps Necko shouldn't be the thing maintaining memory caches, but rather we should have some higher-level non-network component, layed on top of Necko, managing any memory caches in content-dependent formats. As I mentioned in bug 717761 [1], it doesn't seem right to me that our cache operations bounce between the main thread, the cache thread, and the socket transport thread so much, potentially spending a lot of time during each "bounce" waiting for unrelated events in the target thread's event queue to complete. In some cases it is effectively like this: Main Thread Cache Thread Socket Transport Thread ------------- ---------------- ----------------------- Post event to cache unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing Process that event unrelated processing disk I/O for event unrelated processing Post event to main unrelated processing process event from cache Ask cache something unrelated I/O wait unrelated I/O wait unrelated I/O wait unrelated I/O wait compute answer get answer from cache Ask cache something unrelated I/O wait unrelated I/O wait unrelated I/O wait compute answer get answer from cache Ask cache something unrelated I/O wait unrelated I/O wait unrelated I/O wait compute answer get answer from cache compute entry freshness ask network to revalidate entry unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing send request unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing send response to main thread unrelated processing process 304 response ask cache for cached response body unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing return response body unrelated processing unrelated processing process response body It seems to me that we should make an incremental improvement so that it looks more like this: Main Thread Cache Thread Socket Transport Thread ------------- ---------------- ----------------------- Post event to cache unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing Process that event unrelated processing disk I/O for event unrelated processing compute entry freshness unrelated processing ask network to unrelated processing revalidate entry unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing send request unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing send response to main thread unrelated processing process 304 response ask cache for cached response body unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing unrelated I/O unrelated processing return response body unrelated processing unrelated processing process response body And, in parallel to that effort, work on de-serializing I/O in the cache, and/or prioritizing I/O for the cache (e.g. so that I/O for loading things currently visible on the active tab in the active window can budge in the I/O queue) so that it ends up more like this: Main Thread Cache Thread Socket Transport Thread ------------- ---------------- ----------------------- Post event to cache unrelated I/O unrelated processing Process that event unrelated processing disk I/O for event unrelated processing compute entry freshness unrelated processing ask network to unrelated processing revalidate entry unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing send request unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing send response to main thread unrelated processing process 304 response ask cache for cached response body unrelated I/O unrelated processing return response body unrelated processing unrelated processing process response body And then finally we could remove the main thread interaction in the middle: Main Thread Cache Thread Socket Transport Thread ------------- ---------------- ----------------------- Post event to cache unrelated I/O unrelated processing Process that event unrelated processing disk I/O for event unrelated processing compute entry freshness unrelated processing ask network to unrelated processing revalidate entry unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing send request unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing unrelated socket I/O unrelated processing send response to cache thread unrelated processing unrelated I/O unrelated processing Process 304 response unrelated processing read cached body unrelated processing return cached body to main thread unrelated processing unrelated processing process response body When we don't need to revalidate an entry, then this would collapse to: Main Thread Cache Thread ------------- ---------------- Post event to cache unrelated I/O unrelated processing Process that event unrelated processing disk I/O for event unrelated processing compute entry freshness unrelated processing read cached body unrelated processing return cached body to main thread unrelated processing unrelated processing process response body Thoughts? - Brian [1] https://bugzilla.mozilla.org/show_bug.cgi?id=717761#c1 [2] https://bugzilla.mozilla.org/show_bug.cgi?id=695399 [3] https://bugzilla.mozilla.org/show_bug.cgi?id=660749 _______________________________________________ dev-tech-network mailing list [email protected] https://lists.mozilla.org/listinfo/dev-tech-network
