We haven't changed the ceiling MaxRequestWorkers to 1024 yet. Our targeted number is about 800~900 concurrent users attempt the online quiz. We have to add the MPM directive carefully. Maybe need to wait for opportunity of testing at the next online exam.
Yes, our server should be able to support 400 concurrent users - the default MaxRequestWorkers value, but it was overloaded with 350. I think the reason is that each student submitted about 5 jpg files to the Moodle. Data was not fast enough to be written to hard disk, student might upload the second jpg file without waiting for the completion of the previous upload. This led to parallel http post request. That is why the error message "cannot obtain session lock for sid" appeared at the same time. This act behaved like multiplication effect of the number of concurrent users. 350x5 = 1750.
Perhaps, the http request was queuing up initially. When certain amount of "invalid login token" accumulated, it reached the limit of MaxRequestWorkers. Therefore, we saw the error message "consider raising the MaxRequestWorkers setting" accompanying by the error message of "cannot obtain session lock..".
Yes, our server should be able to support 400 concurrent users - the default MaxRequestWorkers value, but it was overloaded with 350. I think the reason is that each student submitted about 5 jpg files to the Moodle. Data was not fast enough to be written to hard disk, student might upload the second jpg file without waiting for the completion of the previous upload. This led to parallel http post request. That is why the error message "cannot obtain session lock for sid" appeared at the same time. This act behaved like multiplication effect of the number of concurrent users. 350x5 = 1750.
Perhaps, the http request was queuing up initially. When certain amount of "invalid login token" accumulated, it reached the limit of MaxRequestWorkers. Therefore, we saw the error message "consider raising the MaxRequestWorkers setting" accompanying by the error message of "cannot obtain session lock..".