I was investigating a severe performance difference between cp and xcopy of large files from/to local disk, and found the following:
via filemon: xcopy uses 64k I/Os cmd copy uses 64k I/Os explorer uses 64k I/Os cp uses 8k I/Os (coreutils-6.7-2) cp uses 1k I/Os (coreutils-5.3.0-9) The 8k in cp (coreutils-6.7-2) comes from a built in floor in the copy.c code not present in a previous cp (coreutils-5.3.0-9). Typical timings for a 28M file produced via: dd if=/dev/zero of=28Mfile bs=28M count=1 are: $ time cp-5-3-0-9 28Mfile dst real 0m3.063s user 0m0.640s sys 0m2.233s $ time cp-6-7-2 28Mfile dst real 0m2.625s user 0m0.061s sys 0m0.358s I'm not quite sure how to account for the real time difference, but I assure you it is real and repeatable. $ time xcopy 28Mfile dst C:28Mfile 1 File(s) copied real 0m0.547s user 0m0.015s sys 0m0.015s as a sanity check: $ time dd if=28Mfile of=dst/28Mfile bs=1k 28672+0 records in 28672+0 records out 29360128 bytes (29 MB) copied, 3.64 seconds, 8.1 MB/s real 0m3.687s user 0m0.468s sys 0m2.327s $ time dd if=28Mfile of=dst/28Mfile bs=8k 3584+0 records in 3584+0 records out 29360128 bytes (29 MB) copied, 2.562 seconds, 11.5 MB/s real 0m2.609s user 0m0.124s sys 0m0.499s Likewise missing real time that is repeatable. $ time dd if=28Mfile of=dst/28Mfile bs=64k 448+0 records in 448+0 records out 29360128 bytes (29 MB) copied, 0.875 seconds, 33.6 MB/s real 0m0.938s user 0m0.061s sys 0m0.093s Ah, much closer! So, it does indeed appear to be I/O size dependent. Current cp gets its initial buffer size from the st_blksize field returned by fstat, which is currently set to 1k. I have found several references on the web that state 64k is the "native" I/O size for the disk cache manager. This might also fix the infamous USB transfer rate issue as discussed here: http://www.cygwin.com/ml/cygwin/2006-08/msg00090.html Should we change st_blksize to 64k for performance reasons? The benefit is non-trivial, IMHO. If so, what scope should this change be (for disk files, raw devices, mail slots, etc.) (memory devices already use 64k for st_blksize)? Thanks. -- Brian Ford Lead Realtime Software Engineer VITAL - Visual Simulation Systems FlightSafety International the best safety device in any aircraft is a well-trained crew... -- Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple Problem reports: http://cygwin.com/problems.html Documentation: http://cygwin.com/docs.html FAQ: http://cygwin.com/faq/
