• Steven Rostedt (VMware)'s avatar
    ring-buffer: Return reader page back into existing ring buffer · 73a757e6
    Steven Rostedt (VMware) authored
    When reading the ring buffer for consuming, it is optimized for splice,
    where a page is taken out of the ring buffer (zero copy) and sent to the
    reading consumer. When the read is finished with the page, it calls
    ring_buffer_free_read_page(), which simply frees the page. The next time the
    reader needs to get a page from the ring buffer, it must call
    ring_buffer_alloc_read_page() which allocates and initializes a reader page
    for the ring buffer to be swapped into the ring buffer for a new filled page
    for the reader.
    
    The problem is that there's no reason to actually free the page when it is
    passed back to the ring buffer. It can hold it off and reuse it for the next
    iteration. This completely removes the interaction with the page_alloc
    mechanism.
    
    Using the trace-cmd utility to record all events (causing trace-cmd to
    require reading lots of pages from the ring buffer, and calling
    ring_buffer_alloc/free_read_page() several times), and also assigning a
    stack trace trigger to the mm_page_alloc event, we can see how many times
    the ring_buffer_alloc_read_page() needed to allocate a page for the ring
    buffer.
    
    Before this change:
    
      # trace-cmd record -e all -e mem_page_alloc -R stacktrace sleep 1
      # trace-cmd report |grep ring_buffer_alloc_read_page | wc -l
      9968
    
    After this change:
    
      # trace-cmd record -e all -e mem_page_alloc -R stacktrace sleep 1
      # trace-cmd report |grep ring_buffer_alloc_read_page | wc -l
      4
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    73a757e6
ring_buffer_benchmark.c 10.7 KB