On Wed, 2011-12-14 at 11:43 -0700, Mark Post wrote: > >>> On 12/14/2011 at 07:20 AM, Jan Glauber <[email protected]> wrote: > > > I can easily replace the mmap-memcpy with pread/pwrite. Unfortunately I > > see huge performance drops if I do so. Currently I'm looking why the > > system call variant costs so much more than mmap. > > Define "huge performance drops." I don't know that most people are going to > be reading large files from a CMS minidisk. They're more likely to have > large disks with lots of reasonably sized files. I suspect that some people > are wanting to read very large files, but I don't know how many people, or > how large the files are.
I've seen read performance go down by a factor of ten by using pread vs. memcpy. But this measurement seems not worth the bytes I'm currently wasting. Today everything is fast (enough) again, regardless of pread or memcpy. I suspect there was something fishy with the disk I'm using. While I would not like to introduce something that costs factor of 10 I think CMS EDF is not a high-performance file system and neither is cmsfs-fuse for various reasons. > Would it be possible to use pread for the FST, and then mmap-memcpy the > actual file being read? I would find it far more understandable to need more > memory to read a very large file than I would to just mount the file system. That would not be possible since files (and also FST's and the allocation map) could consist of (nearly) any block on the disk so they are not contiguous. And mmap'ing single blocks would not be worth the effort. Jan > > Mark Post > > ---------------------------------------------------------------------- > For LINUX-390 subscribe / signoff / archive access instructions, > send email to [email protected] with the message: INFO LINUX-390 or visit > http://www.marist.edu/htbin/wlvindex?LINUX-390 > ---------------------------------------------------------------------- > For more information on Linux on System z, visit > http://wiki.linuxvm.org/ > ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
