On 9/25/2020 12:17 PM, Amy Smith via
openssl-users wrote:
I mmap the file which server will send (to eliminate disk performance).
In general, this won't work. In fact, it may make it *harder* to eliminate disk performance from a measurement.
First, of course, mmap() isn't magic. The data has to be read off the disk at some point. The only question is when.
Second, mmap() does not necessarily (and probably does not) read the data immediately. It sets up the paging tables so that those areas of virtual memory are paged to/from the corresponding parts of the file. Just as for a page that has been kicked out of RAM into swap space, the first access to a page causes the page to be read.
If you do a plain read() then you can be pretty confident that when the read() returns the data has been read off the disk and into RAM. The same is *not* true of mmap(). If you mmap() a file and then start to access it, the disk I/O will be in-line with your accesses.
mmap() may eliminate a memory-to-memory copy of the data, since read() probably does not do the disk I/O directly into your process's buffer, but mmap() might. (Or, a bit more precisely, your mapping might point to the same page that the kernel is using for its disk buffer.)
Of course any particular implementation could do things a bit differently, but that's my perception of how it's usually done.
-- Jordan Brown, Oracle ZFS Storage Appliance, Oracle Solaris
