Andy Polyakov wrote:
as you still have to pull a lot of data from disk, you still put
quite a pressure on VM subsystem, so direct I/O can still help,
But how to talk afio, star or mkisofs into that ?
I was thinking that a simple wrapper to open() which adds
O_DIRECT might be sufficient, but it turned out that this
alone is not sufficient: the buffers used by the programs
must have a certain alignment. This is not guaranteed
without modifying the way how those programs allocate
memory.
Do note that I assert that it *can* help. In particular if it
eliminates need for substantial amount of redundant page faults, most
notably other applications' working sets from being offloaded to swap
partition because of pressure on the block buffer and repeatedly
requested back during recording [with modern bloated GUI it's more
than common scenario], or some frequently used portions of block
buffer from being purged. "Frequently used" means "multiple times
during recording, but not frequent enough for raging data stream not
to repeatedly kick it out." As was pointed out by Bill if you want
*guaranteed* success with direct I/O, you have to complement your code
with some asynchronism, e.g. you at least want to partially undertake
the block buffer role and perform pre-fetch [and pre-open per Bill's
suggestion]. Once again, growisofs attempts to take advantage of
direct I/O for "image" recordings. "Attempts" means that it passes the
O_DIRECT flag to the kernel and aligns buffers, but I can't give any
guarantees that some particular kernel actually respects the flag.
Test it, provide feedback...
My belief is that if O_DIRECT is in the kernel headers it works. I would
love to have time to test timing of O_DIRECT on (a) disk, (b) partition,
(c) file on disk, and alignment on minimal vs. page size boundaries.
Doing O_DIRECT writes of anything avoids buffer pool collisions. To test
either hack mkisofs to write with O_DIRECT or pipe into Dwriter or
similar and see the difference in create time of a DVD image. I may
actually do the hack and enable it when -o is used (and on Linux). If I
do I'll put up the patch and post a link here. Then Joerg can reject it
because it makes mkisofs run faster on Linux.
Hum, have to put Dwriter and a doc file on my web site with the other
tools :-(
On a side note it should also be noted that in particular Linux
requirements for alignment are not really reasonable. Minimal
alignment required by DMA controller, cache line size to be specific,
should suffice, but they've chosen to insist on block or sector
alignment. A.
--
bill davidsen <[EMAIL PROTECTED]>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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