Hello! I'm using libcurl in a C++ service for reaching out to a couple of other services. My C++ service is a standard service - it listens for requests, then uses libcurl to call a couple of other services, processes the responses and returns something back to the caller.
I have been using HTTP so far with libcurl and have been able to scale up to serve ~1000 incoming requests per second, per host for my service, with an average latency of 10ms and a max latency of 80ms. When I switched to HTTPS I was not able to even serve 500 requests per second, because of some bottlenecks in my service implementation. I was able to figure out some of these bottlenecks and fix them (one of these was not setting up proper connection re-use with libcurl), and get up to 700 requests per second. However, with 700 requests I'm still seeing worse latencies which average at 10ms but sometimes go up to 1 second or more for the worst 1% of requests. I did some profiling and noticed calls to the curl_multi_socket_action method sometimes take 50 or 100 ms, which does not happen when using HTTP. These long calls are adding up and causing a delay in processing the requests that came in while we were processing pending requests with curl. I've read a few different messages on this forum as well as noticed that the "mastering libcurl" videos reference that libcurl can scale up to serve 40k or 50k transfers at once. Can someone help me out with what could be causing the curl_multi_socket_action method to take more time specifically for HTTPS requests? Does the curl_multi_socket_action perform TLS handshakes? And, are there some specific things I should configure to ensure my integration with libcurl is able to scale well? Here's some specifics about my setup: - OS: Linux (kernel: 4.9) - libcurl version: 7.72 - TLS backend: OpenSSL, version: 1.0.2 - I'm using the curl multi interface with epoll, my implementation matches the ephiper example here: https://curl.se/libcurl/c/ephiperfifo.html Here's a list of things I've tried/looked at: - Increasing the connection pool size (by setting a hard count for CURLMOPT_MAXCONNECTS, I'm currently using 2000 and I increased this to 4000 but observed no impact on my latencies) - Measured the time it takes for my socket callback method (configured using CURLMOPT_SOCKETFUNCTION), this is less than 50ms always - Checked the time it takes for TLS handshakes using data from tcpdump - this is around 8ms - Logged the connect and app connect times. The connect time spikes up to 500 ms and the app connect time spikes up to 1.25 - 1.5 seconds. The spikes in these two align with the spikes I'm observing for the service latencies as well. Thanks, Richa
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