208 lines
4.1 KiB
HTML
208 lines
4.1 KiB
HTML
<HTML
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>Mmap-ed IO</TITLE
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NAME="GENERATOR"
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HREF="index.html"><LINK
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TITLE="Direct and Mmap-ed IO"
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>The Linux SCSI Generic (sg) HOWTO</TH
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><TD
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>Chapter 9. Direct and Mmap-ed IO</TD
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="MMAPIO">9.2. Mmap-ed IO</H1
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><P
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>Memory-mapped IO takes a different approach from direct IO to removing
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the extra data copy performed by normal ("indirect") IO. With mmap-ed
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IO the application calls the mmap() system call to memory map sg's reserved
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buffer. The sg driver maintains one reserved buffer per file descriptor.
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The default size of the reserved buffer is 32 KB and it can be changed
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with the ioctl(SG_SET_RESERVED_SIZE). The mmap() system call only needs
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to be called once prior
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<A
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NAME="AEN711"
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HREF="#FTN.AEN711"
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>[1]</A
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>
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to doing mmap-ed IO. For more details on the mmap() see
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<A
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HREF="mmap.html"
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>Section 7.6</A
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>. An application indicates that it wants mmap-ed on
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a SCSI request by setting the SG_FLAG_MMAP_IO value in 'flags'.</P
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><P
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>Since there is only reserved buffer per sg file descriptor then only one
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mmap-ed IO command can be active at one time. In order to perform command
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queuing with mmap-ed IO, an application will need to open() multiple
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file descriptors to the same SCSI device.
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With mmap-ed IO the various status values and the sense buffer (if
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required) are conveyed back to an application in the same fashion as
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normal ("indirect") IO.</P
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><P
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>Mmap-ed has very low per command latency since the reserved buffer mapping
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only needs to be done once per file descriptor. Also the reserved buffer
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is set up by the sg driver to aid the efficient construction of the
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internal scatter gather list used by the lower level (adapter) driver for
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DMA purposes. This tends to be more efficient than the user memory that
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direct IO requires the sg driver to process into an internal scatter
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gather list. So on both these counts, mmap-ed IO has the edge over direct IO.</P
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></DIV
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><H3
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CLASS="FOOTNOTES"
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>Notes</H3
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><TABLE
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WIDTH="5%"
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><A
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NAME="FTN.AEN711"
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HREF="mmapio.html#AEN711"
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>[1]</A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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>When a write() or ioctl(SG_IO) attempts mmap-ed IO there is no check
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performed that a prior mmap() system call has been performed. If no
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mmap() has been issued then random data is written to the device or data
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read from the device in inaccessible. Also once mmap() has been called
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on a file descriptor then all subsequent calls to ioctl(SG_SET_RESERVED_SIZE)
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will yield EBUSY.</P
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></TD
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>Prev</A
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>Direct IO</TD
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><TD
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WIDTH="34%"
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VALIGN="top"
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>Up</A
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WIDTH="33%"
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VALIGN="top"
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>Driver and module initialization</TD
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></TR
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> |