On 28/03/2015 19:54, Issac Goldstand wrote:
sendfile is much more efficient than that. At the most basic level,
sendfile allows a file to be streamed directly from the block device (or
OS cache) to the network, all in kernel-space (see sendfile(2)).
What you describe below is less effective, since you need to ask the
kernel to read the data, chunk-by-chunk, send it to userspace, and then
from userspace back to kernel space to be sent to the net.
Beyond that, the Apache output filter stack is also spending time
examining your data, possibly buffering it differently than you are (for
example to make HTTP chunked-encoding) - by using sendfile, you'll be
bypassing the output filter chain (for the request, at least;
connection/protocol filters, such as HTTPS encryption will still get in
the way, but you probably want that to happen :)) further optimizing the
output.
If you're manipulating data, you need to stream yourself, but if you
have data on the disk and can serve it as-is, sendfile will almost
always perform much, much, much better.
In the cases I was pointing out (in line with the original request) that
streaming the
data is more efficient that writing it to disk and then using
sendfile.... (reading is
more efficient than writing - from experience)
Depends on the cost of producing the file - the end time response for
the user may
well be less by streaming it out than writing it to disk - and then
sending it to the
user with sendfile - they will already have most of the file (network
permitting)
before the last chunk of content is produced....
I won't re-iterate the rest of the issues with memory management that
are achieved
but also I will also point out that if you ever write a file to disk you
are putting your
server at risk - either from a security or dos attack point of view - so
if you can avoid
it - do so!
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