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From Ed Korthof ...@organic.com>
Subject [PROPOSAL] Stacked I/O With BUFFs
Date Sun, 18 Jan 1998 00:38:43 GMT
This past summer, Alexei and I wrote a spec for an I/O Filters API... 
this proposal addresses one part of that -- 'stacked' I/O with buff.c. 

We have a couple of options for stacked I/O: we can either use existing
code, such as sfio, or we can rewrite buff.c to do it.  We've gone over
the first possibility at length, though, and there were problems with each
implemenation which was mentioned (licensing and compatibility,
specifically); so far as I know, those remain issues. 

So anyway, this addresses the efficiency concerns which were brought up
the last time we last talked about this on new-httpd. System-specific code
will be needed for Win32 as well as (possibly) for sending files directly
from the disk to the network (zero-copy tcp)  -- if anyone can suggest
specific references on these, I'd love to see them (Alexei has given me
some info on this, but I'd need more to implement them).

Also, I've made the possibly incorrect assumption that we can, one way or
another, efficiently treat data from a file (in Windows) as a constant
char * in function calls.  If that's not the case, what could we use? 
Again, references would be appriciated.

If folks are satisfied with this and want to go the route of modifying
buff.c to provide stacked I/O, I'd like to start implementing it ASAP. 
Btw -- sfio will be supported w/in this model... it just wouldn't be the
basis for the model's implementation. 

     -- Ed Korthof        |  Web Server Engineer --
     -- ed@organic.com    |  Organic Online, Inc --
     -- (415) 278-5676    |  Fax: (415) 284-6891 --

---------------------------------------------------------------------------
Stacked I/O With BUFFs
	Sections:

	1.) Overview
	2.) The API
		User-supplied structures
		API functions
	3.) Detailed Description
		The bfilter structure
		The bbottomfilter structure
		The BUFF structure
		Public functions in buff.c
	4.) Efficiency Considerations
		Buffering
		Memory copies
		Function chaining
		writev
	5.) Code in buff.c
		Default Functions
		Heuristics for writev
		Writing
		Reading
		Flushing data
		Closing stacks and filters
		Flags and Options

*************************************************************************
		Overview

The intention of this API is to make Apache's BUFF structure modular
while retaining high efficiency.  Basically, it involves rewriting
buff.c to provide 'stacked' I/O -- where the data passed through a
series of 'filters', which may modify it.

There are two parts to this, the core code for BUFF structures, and the
"filters" used to implement new behavior.  "filter" is used to refer to
both the sets of 5 functions, as shown in the bfilter structure in the
next section, and to BUFFs which are created using a specific bfliter.
These will also be occasionally refered to as "user-supplied", though
the Apache core will need to use these as well for basic functions.

The user-supplied functions should use only the public BUFF API, rather
than any internal details or functions.  One thing which may not be
clear is that in the core BUFF functions, the BUFF pointer passed in
refers to the BUFF on which the operation will happen.  OTOH, in the
user-supplied code, the BUFF passed in is the next buffer down the
chain, not the current one.

*************************************************************************
		The API

	User-supplied structures

First, the bfilter structure is used in all filters:
    typedef struct {
      int (*writev)(BUFF *, void *, struct iovect *, int);
      int (*read)(BUFF *, void *, char *, int);
      int (*write)(BUFF *, void *, const char *, int);
      void (*flush)(BUFF *, void *);
      void (*close)(BUFF *, void *);
    } bfilter;

bfilters are placed into a BUFF structure along with a
user-supplied void * pointer.

Second, the following structure is for use with a filter which can
sit at the bottom of the stack:

    typedef struct {
      void *(*bgetfileinfo)(BUFF *, void *);
      void (*bpushfileinfo)(BUFF *, void *, void *);
    } bbottomfilter;


	BUFF API functions

The following functions are new BUFF API functions:

For filters:

BUFF * bcreatestack(pool *p, int flags, struct bfilter *,
                    struct bbottomfilter *, void *);
BUFF * bpushfilter (BUFF *, struct bfilter *, void *);
BUFF * bpushbuffer (BUFF *, BUFF *);
BUFF * bpopfilter(BUFF *);
BUFF * bpopbuffer(BUFF *);
void bclosestack(BUFF *);

For BUFFs in general:

int bsetstackopts(BUFF *, int, const void *);
int bsetstackflags(BUFF *, int, int);

Note that a new flag is needed for bsetstackflags:
B_MAXBUFFERING

The current bcreate should become

BUFF * bcreatebuffer (pool *p, int flags, struct bfilter *, void *);

*************************************************************************
		Detailed Explanation

	bfilter structure

The void * pointer used in all these functions, as well as those in the
bbottomfilter structure and the filter API functions, is always the same
pointer w/in an individual BUFF.

The first function in a bfilter structure is 'writev'; this is only
needed for high efficiency writing, generally at the level of the system
interface.  In it's absence, multiple writes will be done w/ 'write'.
Note that defining 'writev' means you must define 'write'.

The second is 'write'; this is the generic writing function, taking a BUFF
* to which to write, a block of text, and the length of that block of
text.  The expected return is the number of characters (out of that block
of text) which were successfully processed (rather than the number of
characters actually written). 

The third is 'read'; this is the generic reading function, taking a BUFF *
from which to read data, and a void * buffer in which to put text, and the
number of characters to put in that buffer.  The expected return is the
number of characters placed in the buffer.

The fourth is 'flush'; this is intended to force the buffer to spit out
any data it may have been saving.  Note that general buffering is handled
by BUFF's internal code, and module writers should not store data for
performance reasons.

The fifth is 'close'; this is what is called when the connection is being
closed.  The 'close' should not be passed on to the next filter in the
stack.  Most filters will not need to use this, but if database handles
or some other object is created, this is the point at which to remove it.
Note that flush is called automatically before this.

	bbottomfilter Structure

The first function, bgetfileinfo, is designed to allow Apache to get
information from a BUFF struct regarding the input and output sources.
This is currently used to get the input file number to select on a
socket to see if there's data waiting to be read.  The information
returned is platform specific; the void * pointer passed in holds
the void * pointer passed to all user-supplied functions.

The second function, bpushfileinfo, is used to push file information
onto a buffer, so that the buffer can be fully constructed and ready
to handle data as soon as possible after a client has connected.
The first void * pointer holds platform specific information (in
Unix, it would be a pair of file descriptors); the second holds the
void * pointer passed to all user-supplied functions.


	The BUFF structure

A couple of changes are needed for this structure: remove fd and
fd_in; add a bfilter structure; add a pointer to a bbottomfilter;
add three pointers to the next BUFFs: one for the next BUFF in the
stack, one for the next BUFF which implements write, and one
for the next BUFF which implements read.


	Public functions in buff.c

BUFF * bpushfilter (BUFF *, struct bfilter *, void *);

This function adds the filter functions from bfilter, stacking them on
top of the BUFF.  It returns the new top BUFF, or NULL on error.

BUFF * bpushbuffer (BUFF *, BUFF *);

This function places the second buffer on the top of the stack that
the first one is on.  It returns the new top BUFF, or NULL on error.

BUFF * bpopfilter(BUFF *);
BUFF * bpopbuffer(BUFF *);

Unattaches the top-most filter from the stack, and returns the new
top-level BUFF, or NULL on error or when there are no BUFFs
remaining.  The two are synonymous.

void bclosestack(BUFF *);

Closes the I/O stack, removing all the filters in it.

BUFF * bcreatestack(pool *p, int flags, struct bfilter *,
                    struct bbottomfilter *, void *);

This creates an I/O stack.  It returns NULL on error.

BUFF * bcreatebuffer(pool *p, int flags, struct bfilter *, void *);

This creates a BUFF for later use with bpushbuffer.  The BUFF is
not set up to be used as an I/O stack, however.  It returns NULL
on error.

int bsetstackopts(BUFF *, int, const void *);
int bsetstackflags(BUFF *, int, int);

These functions, respectively, set options on all the BUFFs in a
stack.  The new flag, B_MAXBUFFERING is used to disable a feature
described in the next section, whereby only the first and last
BUFFs will buffer data.

*************************************************************************
		Efficiency Considerations

	Buffering

All input and output is buffered by the standard buffering code.
People writing code to use buff.c should not concern themselves with
buffering for efficiency, and should not buffer except when necessary.

The write function will typically be called with large blocks of text;
the read function will attempt to place the specified number of bytes
into the buffer.

Dean noted that there are possible problems w/ multiple buffers;
further, some applications must not be buffered.  This can be
partially dealt with by turning off buffering, or by flushing the
data when appropriate.

However, some potential problems arrise anyway.  The simplest example
involves shrinking transformations; suppose that you have a set
of filters, A, B, and C, such that A outputs less text than it
recieves, as does B (say A strips comments, and B gzips the result).
Then after a write to A which fills the buffer, A writes to B.
However, A won't write enough to fill B's buffer, so a memory copy
will be needed.  This continues till B's buffer fills up, then
B will write to C's buffer -- with the same effect.

The maximum additional number of bytes which will be copied in this
scenario is on the order of nk, where n is the total number of bytes,
and k is the number of filters doing shrinking transformations.

There are several possible solutions to this issue.  The first
is to turn off buffering in all but the first filter and the
last filter.  This reduces the number of unnecessary byte copies
to at most one per byte, however it means that the functions in
the stack will get called more frequently; but it is the default
behavior, overridable by setting the B_MAXBUFFERING with
bsetstackflags.  Most filters won't involve a net shrinking
transformation, so even this will rarely be an issue; however,
if the filters do involve a net shrinking transformation, for
the sake of network-efficiency (sending reasonably sized blocks),
it may be more efficient anyway.

A second solution is more general use of writev for communication
between different buffers.  This complicates the programing work,
however.


	Memory copies

Each write function is passed a pointer to constant text; if any changes
are being made to the text, it must be copied.  However, if no changes
are made to the text (or to some smaller part of it), then it may be
sent to the next filter without any additional copying.  This should
provide the minimal necessary memory copies.

	Function chaining

In order to avoid unnecessary function chaining for reads and writes,
when a filter is pushed onto the stack, the buff.c code will determine
which is the next BUFF which contains a read or write function, and
reads and writes, respectively, will go directly to that BUFF.

	writev

writev is a function for efficient writing to the system; in terms of
this API, however, it also works for dealing with multiple blocks of
text without doing unnecessary byte copies.  It is not required.

Currently, the system level writev is used in two contexts: for
chunking and when a block of text is writen which, combined with
the text already in the buffer, would make the buffer overflow.

writev would be implemented both by the default bottom level filter
and by the chunking filter for these operations.  In addition, writev
may, be used, as noted above, to pass multiple blocks of text w/o
copying them into a single buffer.  Note that if the next filter does
not implement writev, however, this will be equivalent to repeated
calls to write, which may or may not be more efficient.  Up to
IOV_MAX-2 blocks of text may be passed along in this manner.  Unlike
the system writev call, the writev in this API should be called only
once, with a array with iovec's and a count as to the number of
iovecs in it.

If a bfilter defines writev, writev will be called whether or not
NO_WRITEV is set; hence, it should deal with that case in a reasonable
manner.

*************************************************************************
		Code in buff.c

	Default Functions

The default actions are generally those currently performed by Apache,
save that they they'll only attempt to write to a buffer, and they'll
return an error if there are no more buffers.  That is, you must implement
both read and write in the bottom-most filter.

Except for close(), the default code will simply pass the function call
on to the next filter in the stack.  Some samples follow.

	Heuristics for writev

Currently, we call writev for chunking, and when we get a enough so that
the total overflows the buffer.  Since chunking is going to become a
filter, the chunking filter will use writev; in addition, bwrite will
trigger bwritev as shown (note that system specific information should
be kept at the filter level):

in bwrite:

    if (fb->outcnt > 0 && nbyte + fb->outcnt >= fb->bufsiz) {
        /* build iovec structs */
        struct iovec vec[2];
        vec[0].iov_base = (void *) fb->outbase;
        vec[0].iov_len = fb->outcnt;
        fb->outcnt = 0;
        vec[1].iov_base = (void *)buff;
        vec[1].iov_length = nbyte;
        return bwritev (fb, vec, 2);
    } else if (nbye >= fb->bufsiz) {
        return write_with_errors(fb,buff,nbyte);
    }

Note that the code above takes the place of large_write (as well
as taking code from it).

So, bwritev would look something like this (copying and pasting freely
from the current source for writev_it_all, which could be replaced):

-----
int bwritev (BUFF * fb, struct iovec * vec, int nvecs) {
    if (!fb)
        return -1; /* the bottom level filter implemented neither write nor
                    * writev. */
    if (fb->bfilter.bwritev) {
        return bf->bfilter.writev(fb->next, vec, nvecs);
    } else if (fb->bfilter.write) {
        /* while it's nice an easy to build the vector and crud, it's painful
         * to deal with partial writes (esp. w/ the vector)
         */
        int i = 0,rv;
        while (i < nvecs) {
            do {
                rv = fb->bfilter.write(fb, vec[i].iov_base, vec[i].iov_len);
            } while (rv == -1 && (errno == EINTR || errno == EAGAIN)
                     && !(fb->flags & B_EOUT));
            if (rv == -1) {
                if (errno != EINTR && errno != EAGAIN) {
                    doerror (fb, B_WR);
                }
                return -1;
            }
            fb->bytes_sent += rv;
            /* recalculate vec to deal with partial writes */
            while (rv > 0) {
                if (rv < vec[i].iov_len) {
                    vec[i].iov_base = (char *)vec[i].iov_base + rv;
                    vec[i].iov_len -= rv;
                    rv = 0;
                    if (vec[i].iov_len == 0) {
                        ++i;
                    }
                } else {
                    rv -= vec[i].iov_len;
                    ++i;
                }
            }
            if (fb->flags & B_EOUT)
                return -1;
        }
        /* if we got here, we wrote it all */
        return 0;
    } else {
        return bwritev(fb->next,vec,nvecs);
    }
}
-----
The default filter's writev function will pretty much like
writev_it_all.


	Writing

The general case for writing data is significantly simpler with this
model.  Because special cases are not dealt with in the BUFF core,
a single internal interface to writing data is possible; I'm going
to assume it's reasonable to standardize on write_with_errors, but
some other function may be more appropriate.

In the revised bwrite (which I'll ommit for brievity), the following
must be done:
	check for error conditions
	check to see if any buffering is done; if not, send the data
		directly to the write_with_errors function
	check to see if we should use writev or write_with_errors
		as above
	copy the data to the buffer (we know it fits since we didn't
		need writev or write_with_errors)

The other work the current bwrite is doing is
	ifdef'ing around NO_WRITEV
	numerous decisions regarding whether or not to send chunks

Generally, buff.c has a number of functions whose entire purpose is
to handle particular special cases wrt chunking, all of which could
be simplified with a chunking filter.

write_with_errors would not need to change; buff_write would.  Here
is a new version of it:

-----
/* the lowest level writing primitive */
static ap_inline int buff_write(BUFF *fb, const void *buf, int nbyte)
{
    if (fb->bfilter.write)
        return fb->bfilter.write(fb->next_writer,buff,nbyte);
    else
        return bwrite(fb->next_writer,buff,nbyte);
}
-----


	Reading

One of the basic reading functions in Apache 1.3b3 is buff_read;
here is how it would look within this spec:

-----
/* the lowest level reading primitive */
static ap_inline int buff_read(BUFF *fb, void *buf, int nbyte)
{
    int rv;

    if (!fb)
        return -1; /* the bottom level filter is not set up properly */

    if (fb->bfilter.read)
        return fb->bfilter.read(fb->next_reader,buf,nbyte);
    else
        return bread(fb->next_reader,buff,nbyte);
}
-----
The code currently in buff_read would become part of the default
filter.


	Flushing data

flush will get passed on down the stack automatically, with recursive
calls to bflush.  The user-supplied flush function will be called then,
and also before close is called.  The user-supplied flush should not
call flush on the next buffer.

	Closing Stacks and Filters

When a filter is removed from the stack, flush will be called then close
will be called.  When the entire stack is being closed, this operation
will be done automatically on each filter within the stack; generally,
filters should not operate on other filters further down the stack,
except to pass data along when flush is called.

	Flags and Options

Changes to flags and options using the current functions only affect
one buffer.  To affect all the buffers on down the chain, use
bsetstackopts or bsetstackflags.

bgetopt is currently only used to grab a count of the bytes sent;
it will continue to provide that functionality.  bgetflags is
used to provide information on whether or not the connection is
still open; it'll continue to provide that functionality as well.

The core BUFF operations will remain, though some operations which
are done via flags and options will be done by attaching appropriate
filters instead (eg. chunking).


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