core_output_filter: промывка из-за корзины FLUSH - почему я вижу это в журнале ошибок Apache?

Кто-нибудь знает, почему я продолжаю видеть core_output_filter: flushing because of FLUSH bucket в моем журнале ошибок Apache 2.4.7 (Ubuntu) и что это означает?

Я пока не заметил никаких проблем, но это действительно затрудняет чтение error.log. Журнал все время выглядит так:

    [Mon Apr 23 09:22:34.927673 2018] [core:trace6] [pid 6096] core_filters.c(527): [client 127.0.0.1:40876] core_output_filter: flushing because of FLUSH bucket
    [Mon Apr 23 09:22:34.927793 2018] [core:trace6] [pid 6159] core_filters.c(527): [client 127.0.0.1:40868] core_output_filter: flushing because of FLUSH bucket
    [Mon Apr 23 09:22:34.927892 2018] [core:trace6] [pid 6160] core_filters.c(527): [client 127.0.0.1:40870] core_output_filter: flushing because of FLUSH bucket
    [Mon Apr 23 09:22:35.160482 2018] [core:trace6] [pid 6092] core_filters.c(527): [client 127.0.0.1:40866] core_output_filter: flushing because of FLUSH bucket
    [Mon Apr 23 09:24:07.472559 2018] [core:trace5] [pid 6161] protocol.c(655): [client 127.0.0.1:40888] Request received from client: POST /index.php?module=000100 HTTP/1.1
    [Mon Apr 23 09:24:07.721570 2018] [core:trace6] [pid 6161] core_filters.c(527): [client 127.0.0.1:40888] core_output_filter: flushing because of FLUSH bucket
    [Mon Apr 23 09:24:12.725587 2018] [core:trace6] [pid 6161] core_filters.c(527): [client 127.0.0.1:40888] core_output_filter: flushing because of FLUSH bucket
    [Mon Apr 23 09:29:07.829907 2018] [core:trace5] [pid 6093] protocol.c(655): [client 127.0.0.1:41056] Request received from client: POST /index.php?module=000100 HTTP/1.1
    [Mon Apr 23 09:29:07.986466 2018] [core:trace6] [pid 6093] core_filters.c(527): [client 127.0.0.1:41056] core_output_filter: flushing because of FLUSH bucket
    [Mon Apr 23 09:29:12.990794 2018] [core:trace6] [pid 6093] core_filters.c(527): [client 127.0.0.1:41056] core_output_filter: flushing because of FLUSH bucket