On Sat, Jun 18, 2016 at 11:53 AM, Luca Toscano <toscano.l...@gmail.com> wrote: > Hi Apache devs! > > I have a question for you about the following users@ email thread: > > - > https://lists.apache.org/thread.html/ba26440a53773426e29296569bec17692c77a4a3bd07e8b5331474c4@1464703063@%3Cusers.httpd.apache.org%3E > > This one is about Yann's fix for the MinSpareThreads lower bound calculation > for worker/event: > http://svn.apache.org/viewvc?view=revision&revision=1737447 > > I was able to explain how it works in case of bucket = 1, but I have doubts > when buckets > 1. I've read event/worker code and IIUC: > > 1) min_spare_threads = threads_per_child * (num_buckets - 1) + num_buckets > 2) max_spare_threads = min_spare_threads + threads_per_child * num_buckets > 3) idle_spawn_rate controls the amount of new children created in each > perform_idle_server_maintenance run. > 4) perform_idle_server_maintenance is called per bucket and calculates > max|min_spare_threads accordingly. > > Rick's question in the email thread is about what happens when, with two > buckets and two processes for example, httpd reaches 50% of idle threads. I > made some calculations and an extra process should be indeed created, but > then killed because of the max_spare_threads limit. Is it going to keep > creating/destroying child processes or does idle_spawn_rate prevents it?
Indeed. StartServers = 1 ThreadsPerChild = 4 MinSpareThreads = 1 MaxSpareThreads = 1 ListenCoresBucketRatio = 1 with 2 CPU cores => num_buckets = 2 1. Startup: min_spare_threads = 4 * (2 - 1) + 2 = 6 max_spare_threads = 6 + 4 * 2 = 14 min_spare_threads_per_bucket = min_spare_threads / num_buckets = 3 max_spare_threads_per_bucket = max_spare_threads / num_buckets = 7 idle_thread_count = StartServers * ThreadsPerChild = 4 idle_thread_count_per_bucket = idle_thread_count / num_buckets = 4 / 2 = 2 => idle_thread_count_per_bucket < min_spare_threads_per_bucket => create 1 child => idle_thread_count += ThreadsPerChild => idle_thread_count_per_bucket += ThreadsPerChild / num_buckets => idle_thread_count_per_bucket = 4 => min_spare_threads_per_bucket <= idle_thread_count_per_bucket <= max_spare_threads_per_bucket => 2 children (1 per bucket), fine 2. Two new connections (SO_REUSEPORT garanties even distribution hence one connection per bucket/child): => idle_thread_count_per_bucket -= 2 / num_buckets => idle_thread_count_per_bucket = 4 - 1 = 3 => min_spare_threads_per_bucket <= idle_thread_count_per_bucket <= max_spare_threads_per_bucket => 2 children (1 per bucket), fine 3. Two new connections (total 4): => idle_thread_count_per_bucket -= 2 / num_buckets => idle_thread_count_per_bucket = 3 - 1 = 2 => idle_thread_count_per_bucket < min_spare_threads_per_bucket => create 2 children (the above is true for the 2 existing children) => idle_thread_count += ThreadsPerChild * num_buckets => idle_thread_count_per_bucket += ThreadsPerChild * num_buckets / num_buckets => idle_thread_count_per_bucket = 4 + 4 = 8 => 4 children (2 per bucket), fine 4. Next perform_idle_server_maintenance() round (still 4 connections) => idle_thread_count_per_bucket > max_spare_threads_per_bucket => kill 2 children (the above is true for the 2 existing children) => same state as step 3 with the same number of connections, not fine... So I think Rick is right, we should set max_spare_threads at least to: min_spare_threads + ThreadsPerChild * num_buckets + num_buckets. For the example above, this would give max_spare_threads_per_bucket = 8, so step 3 would remain valid until the number of connections changes. Actually I first implemented this in r1737447 (for consistency with min_spare_threads changes), but finally kept the original formula (can't remember why, should have run the maths as above...). Anyway, fixed in trunk (r1750218, with a comment), thanks Luca and Rick for the follow up! Regards, Yann.
