johansen at sun.com wrote: > On Wed, Sep 09, 2009 at 04:02:15PM -0500, Rich.Brown at sun.com wrote: > >> == Introduction/Background == >> >> Zero-copy (copy avoidance) is essentially buffer sharing >> among multiple modules that pass data between the modules. >> This proposal avoids the data copy in the READ/WRITE path >> of filesystems, by providing a mechanism to share data buffers >> between the modules. It is intended to be used by network file >> sharing services like NFS, CIFS or others. >> >> Although the buffer sharing can be achieved through a few different >> solutions, any such solution must work with File Event Monitors >> (FEM monitors)[1] installed on the files. The solution must >> allow the underlying filesystem to maintain any existing file >> range locking in the filesystem. >> >> The proposed solution provides extensions to the existing VOP >> interface to request and return buffers from a filesystem. The >> buffers are then used with existing VOP_READ/VOP_WRITE calls with >> minimal changes. >> >> >> == Proposed Changes == >> > <...> > > >> == Using the New VOP Interfaces for Zero-copy == >> >> VOP_REQZCBUF()/VOP_RETZCBUF() are expected to be used in conjunction with >> VOP_READ() or VOP_WRITE() to implement zero-copy read or write. >> >> a. Read >> >> In a normal read, the consumer allocates the data buffer and passes it to >> VOP_READ(). The provider initiates the I/O, and copies the data from its >> own cache buffer to the consumer supplied buffer. >> >> To avoid the copy (initiating a zero-copy read), the consumer >> first calls VOP_REQZCBUF() to inform the provider to prepare to >> loan out its cache buffer. It then calls VOP_READ(). After the >> call returns, the consumer has direct access to the cache buffer >> loaned out by the provider. After processing the data, the >> consumer calls VOP_RETZCBUF() to return the loaned cache buffer to >> the provider. >> > <...> > > >> b. Write >> >> In a normal write, the consumer allocates the data buffer, loads the >> data, >> and passes the buffer to VOP_WRITE(). The provider copies the data from >> the consumer supplied buffer to its own cache buffer, and starts the I/O. >> >> To initiate a zero-copy write, the consumer first calls VOP_REQZCBUF() to >> grab a cache buffer from the provider. It loads the data directly to >> the loaned cache buffer, and calls VOP_WRITE(). After the call returns, >> the consumer calls VOP_RETZCBUF() to return the loaned cache buffer to >> the provider. >> > > Just for clarification: this interface only affects pages mapped in the > kernel, correct? I'm trying to understand if this is just for reducing > the number of in-kernel copies, or if this is a userland <-> kernel > zero-copy interface. > > That is correct. This interface is to prevent in-kernel copies and allow buffer sharing between kernel modules (that can be used by in-kernel services like NFS or CIFS). The spec does not define any userland <-> kernel zero-copy interface.
Thanks, Mahesh > Thanks, > > -j >
