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Memory management revisions, take 2

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: Memory management revisions, take 2
Date: 2000-06-22 03:32:19
Message-ID: 7172.961644739@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Attached is a second pass at redesigning backend memory management.
This is basically my proposal of 29 April, updated per the subsequent
discussion and a couple of other things that have occurred to me since
then.  I'm hoping to put this on the front burner pretty soon, so if
you have any gripes, now is a good time...

			regards, tom lane


Proposal for memory allocation fixes, take 2		21-Jun-2000
--------------------------------------------

We know that Postgres has serious problems with memory leakage during
large queries that process a lot of pass-by-reference data.  There is
no provision for recycling memory until end of query.  This needs to be
fixed, even more so with the advent of TOAST which will allow very large
chunks of data to be passed around in the system.  So, here is a proposal.


Background
----------

We already do most of our memory allocation in "memory contexts", which
are usually AllocSets as implemented by backend/utils/mmgr/aset.c.  What
we need to do is create more contexts and define proper rules about when
they can be freed.

The basic operations on a memory context are:

* create a context

* allocate a chunk of memory within a context (equivalent of standard
  C library's malloc())

* delete a context (including freeing all the memory allocated therein)

* reset a context (free all memory allocated in the context, but not the
  context object itself)

Given a chunk of memory previously allocated from a context, one can
free it or reallocate it larger or smaller (corresponding to standard
library's free() and realloc() routines).  These operations return memory
to or get more memory from the same context the chunk was originally
allocated in.

At all times there is a "current" context denoted by the
CurrentMemoryContext global variable.  The backend macro palloc()
implicitly allocates space in that context.  The MemoryContextSwitchTo()
operation selects a new current context (and returns the previous context,
so that the caller can restore the previous context before exiting).

The main advantage of memory contexts over plain use of malloc/free is
that the entire contents of a memory context can be freed easily, without
having to request freeing of each individual chunk within it.  This is
both faster and more reliable than per-chunk bookkeeping.  We already use
this fact to clean up at transaction end: by resetting all the active
contexts, we reclaim all memory.  What we need are additional contexts
that can be reset or deleted at strategic times within a query, such as
after each tuple.


pfree/prealloc no longer depend on CurrentMemoryContext
-------------------------------------------------------

In this proposal, pfree() and prealloc() can be applied to any chunk
whether it belongs to CurrentMemoryContext or not --- the chunk's owning
context will be invoked to handle the operation, regardless.  This is a
change from the old requirement that CurrentMemoryContext must be set
to the same context the memory was allocated from before one can use
pfree() or prealloc().  The old coding requirement is obviously fairly
error-prone, and will become more so the more context-switching we do;
so I think it's essential to use CurrentMemoryContext only for palloc.
We can avoid needing it for pfree/prealloc by putting restrictions on
context managers as discussed below.

We could even consider getting rid of CurrentMemoryContext entirely,
instead requiring the target memory context for allocation to be specified
explicitly.  But I think that would be too much notational overhead ---
we'd have to pass an apppropriate memory context to called routines in
many places.  For example, the copyObject routines would need to be passed
a context, as would function execution routines that return a
pass-by-reference datatype.  And what of routines that temporarily
allocate space internally, but don't return it to their caller?  We
certainly don't want to clutter every call in the system with "here is
a context to use for any temporary memory allocation you might want to
do".  So there'd still need to be a global variable specifying a suitable
temporary-allocation context.  That might as well be CurrentMemoryContext.


Additions to the memory-context mechanism
-----------------------------------------

If we are going to have more contexts, we need more mechanism for keeping
track of them; else we risk leaking whole contexts under error conditions.

We can do this by creating trees of "parent" and "child" contexts.  When
creating a memory context, the new context can be specified to be a child
of some existing context.  A context can have many children, but only one
parent.  In this way the contexts form a forest (not necessarily a single
tree, since there could be more than one top-level context).

We then say that resetting or deleting any particular context resets or
deletes all its direct and indirect children as well.  This feature allows
us to manage a lot of contexts without fear that some will be leaked; we
only need to keep track of one top-level context that we are going to
delete at transaction end, and make sure that any shorter-lived contexts
we create are descendants of that context.  Since the tree can have
multiple levels, we can deal easily with nested lifetimes of storage,
such as per-transaction, per-statement, per-scan, per-tuple.

For convenience we will also want operations like "reset/delete all
children of a given context, but don't reset or delete that context
itself".


Top-level contexts
------------------

There will be several top-level contexts --- these contexts have no parent
and will be referenced by global variables.  At any instant the system may
contain many additional contexts, but all other contexts should be direct
or indirect children of one of the top-level contexts to ensure they are
not leaked in event of an error.  I presently envision these top-level
contexts:

TopMemoryContext --- allocating here is essentially the same as "malloc",
because this context will never be reset or deleted.  This is for stuff
that should live forever, or for stuff that you know you will delete
at the appropriate time.  An example is fd.c's tables of open files,
as well as the context management nodes for memory contexts themselves.
Avoid allocating stuff here unless really necessary, and especially
avoid running with CurrentMemoryContext pointing here.

PostmasterContext --- this is the postmaster's normal working context.
After a backend is spawned, it can delete PostmasterContext to free its
copy of memory the postmaster was using that it doesn't need.  (Anything
that has to be passed from postmaster to backends will be passed in
TopMemoryContext.  The postmaster will probably have only TopMemoryContext,
PostmasterContext, and possibly ErrorContext --- the remaining top-level
contexts will be set up in each backend during startup.)

CacheMemoryContext --- permanent storage for relcache, catcache, and
related modules.  This will never be reset or deleted, either, so it's
not truly necessary to distinguish it from TopMemoryContext.  But it
seems worthwhile to maintain the distinction for debugging purposes.
(Note: CacheMemoryContext may well have child-contexts with shorter
lifespans.  For example, a child context seems like the best place to
keep the subsidiary storage associated with a relcache entry; that way
we can free rule parsetrees and so forth easily, without having to depend
on constructing a reliable version of freeObject().)

QueryContext --- this is where the storage holding a received query string
is kept, as well as storage that should live as long as the query string,
notably the parsetree constructed from it.  This context will be reset at
the top of each cycle of the outer loop of PostgresMain, thereby freeing
the old query and parsetree.  We must keep this separate from
TopTransactionContext because a query string might need to live either a
longer or shorter time than a transaction, depending on whether it
contains begin/end commands or not.  (This'll also fix the nasty bug that
"vacuum; anything else" crashes if submitted as a single query string,
because vacuum's xact commit frees the memory holding the parsetree...)

TopTransactionContext --- this holds everything that lives until end of
transaction (longer than one statement within a transaction!).  An example
of what has to be here is the list of pending NOTIFY messages to be sent
at xact commit.  This context will be reset, and all its children deleted,
at conclusion of each transaction cycle.  Note: presently I envision that
this context will NOT be cleared immediately upon error; its contents
will survive anyway until the transaction block is exited by
COMMIT/ROLLBACK.  This seems appropriate since we want to move in the
direction of allowing a transaction to continue processing after an error.

StatementContext --- this is really a child of TopTransactionContext,
not a top-level context, but we'll probably store a link to it in a
global variable anyway for convenience.  All the memory allocated during
planning and execution lives here or in a child context.  This context
is deleted at statement completion, whether normal completion or error
abort.

ErrorContext --- this permanent context will be switched into
for error recovery processing, and then reset on completion of recovery.
We'll arrange to have, say, 8K of memory available in it at all times.
In this way, we can ensure that some memory is available for error
recovery even if the backend has run out of memory otherwise.  This should
allow out-of-memory to be treated as a normal ERROR condition, not a FATAL
error.

If we ever implement nested transactions, there may need to be some
additional levels of transaction-local contexts between
TopTransactionContext and StatementContext, but that's beyond the scope of
this proposal.


Transient contexts during execution
-----------------------------------

The planner will probably have a transient context in which it stores
pathnodes; this will allow it to release the bulk of its temporary space
usage (which can be a lot, for large joins) at completion of planning.
The completed plan tree will be in StatementContext.

The executor will have contexts with lifetime similar to plan nodes
(I'm not sure at the moment whether there's need for one such context
per plan level, or whether a single context is sufficient).  These
contexts will hold plan-node-local execution state and related items.
There will also be a context on each plan level that is reset at the start
of each tuple processing cycle.  This per-tuple context will be the normal
CurrentMemoryContext during evaluation of expressions and so forth.  By
resetting it, we reclaim transient memory that was used during processing
of the prior tuple.  That should be enough to solve the problem of running
out of memory on large queries.  We must have a per-tuple context in each
plan node, and we must reset it at the start of a tuple cycle rather than
the end, so that each plan node can use results of expression evaluation
as part of the tuple it returns to its parent node.

By resetting the per-tuple context, we will be able to free memory after
each tuple is processed, rather than only after the whole plan is
processed.  This should solve our memory leakage problems pretty well;
yet we do not need to add very much new bookkeeping logic to do it.
In particular, we do *not* need to try to keep track of individual values
palloc'd during expression evaluation.

Note we assume that resetting a context is a cheap operation.  This is
true already, and we can make it even more true with a little bit of
tuning in aset.c.

There will be some special cases, such as aggregate functions.  nodeAgg.c
needs to remember the results of evaluation of aggregate transition
functions from one tuple cycle to the next, so it can't just discard
all per-tuple state in each cycle.  The easiest way to handle this seems
to be to have two per-tuple contexts in an aggregate node, and to
ping-pong between them, so that at each tuple one is the active allocation
context and the other holds any results allocated by the prior cycle's
transition function.

Executor routines that switch the active CurrentMemoryContext may need
to copy data into their caller's current memory context before returning.
I think there will be relatively little need for that, because of the
convention of resetting the per-tuple context at the *start* of an
execution cycle rather than at its end.  With that rule, an execution
node can return a tuple that is palloc'd in its per-tuple context, and
the tuple will remain good until the node is called for another tuple
or told to end execution.  This is pretty much the same state of affairs
that exists now, since a scan node can return a direct pointer to a tuple
in a disk buffer that is only guaranteed to remain good that long.

A more common reason for copying data will be to transfer a result from
per-tuple context to per-run context; for example, a Unique node will
save the last distinct tuple value in its per-run context, requiring a
copy step.  (Actually, Unique could use the same trick with two per-tuple
contexts as described above for Agg, but there will probably be other
cases where doing an extra copy step is the right thing.)

Another interesting special case is VACUUM, which needs to allocate
working space that will survive its forced transaction commits, yet
be released on error.  Currently it does that through a "portal",
which is essentially a child context of TopMemoryContext.  While that
way still works, it's ugly since xact abort needs special processing
to delete the portal.  Better would be to use a context that's a child
of QueryContext and hence is certain to go away as part of normal
processing.  (Eventually we might have an even better solution from
nested transactions, but this'll do fine for now.)


Mechanisms to allow multiple types of contexts
----------------------------------------------

We may want several different types of memory contexts with different
allocation policies but similar external behavior.  To handle this,
memory allocation functions will be accessed via function pointers,
and we will require all context types to obey the conventions given here.
(This is not very far different from the existing code.)

A memory context will be represented by an object like

typedef struct MemoryContextData
{
    NodeTag        type;           /* identifies exact kind of context */
    MemoryContextMethods methods;
    MemoryContextData *parent;     /* NULL if no parent (toplevel context) */
    MemoryContextData *firstchild; /* head of linked list of children */
    MemoryContextData *nextchild;  /* next child of same parent */
    char          *name;           /* context name (just for debugging) */
} MemoryContextData, *MemoryContext;

This is essentially an abstract superclass, and the "methods" pointer is
its virtual function table.  Specific memory context types will use
derived structs having these fields as their first fields.  All the
contexts of a specific type will have methods pointers that point to the
same static table of function pointers, which will look like

typedef struct MemoryContextMethodsData
{
    Pointer     (*alloc) (MemoryContext c, Size size);
    void        (*free_p) (Pointer chunk);
    Pointer     (*realloc) (Pointer chunk, Size newsize);
    void        (*reset) (MemoryContext c);
    void        (*delete) (MemoryContext c);
} MemoryContextMethodsData, *MemoryContextMethods;

Alloc, reset, and delete requests will take a MemoryContext pointer
as parameter, so they'll have no trouble finding the method pointer
to call.  Free and realloc are trickier.  To make those work, we will
require all memory context types to produce allocated chunks that
are immediately preceded by a standard chunk header, which has the
layout

typedef struct StandardChunkHeader
{
    MemoryContext mycontext;         /* Link to owning context object */
    Size          size;              /* Allocated size of chunk */
};

It turns out that the existing aset.c memory context type does this
already, and probably any other kind of context would need to have the
same data available to support realloc, so this is not really creating
any additional overhead.  (Note that if a context type needs more per-
allocated-chunk information than this, it can make an additional
nonstandard header that precedes the standard header.  So we're not
constraining context-type designers very much.)

Given this, the pfree routine will look something like

    StandardChunkHeader * header = 
        (StandardChunkHeader *) ((char *) p - sizeof(StandardChunkHeader));

    (*header->mycontext->free_p) (p);

We could do it as a macro, but the macro would have to evaluate its
argument twice, which seems like a bad idea (the current pfree macro
does not do that).  This is already saving two levels of function call
compared to the existing code, so I think we're doing fine without
squeezing out that last little bit ...


More control over aset.c behavior
---------------------------------

Currently, aset.c allocates an 8K block for the first allocation in
a context, and doubles that size for each successive block request.
That's good behavior for a context that might hold *lots* of data, and
the overhead wasn't bad when we had only a few contexts in existence.
With dozens if not hundreds of smaller contexts in the system, we will
want to be able to fine-tune things a little better.  I envision the
creator of a context as being able to specify an initial block size and a
maximum block size.  Selecting smaller values will prevent wastage of
space in contexts that aren't expected to hold very much (an example is
the relcache's per-relation contexts).


Other notes
-----------

The original version of this proposal suggested that functions returning
pass-by-reference datatypes should be required to return a value freshly
palloc'd in their caller's memory context, never a pointer to an input
value.  I've abandoned that notion since it clearly is prone to error.
In the current proposal, it is possible to discover which context a
chunk of memory is allocated in (by checking the required standard chunk
header), so nodeAgg can determine whether or not it's safe to reset
its working context; it doesn't have to rely on the transition function
to do what it's expecting.

It might be that the executor per-run contexts described above should
be tied directly to executor "EState" nodes, that is, one context per
EState.  I'm not real clear on the lifespan of EStates or the situations
where we have just one or more than one, so I'm not sure.  Comments?

It would probably be possible to adapt the existing "portal" memory
management mechanism to do what we need.  I am instead proposing setting
up a totally new mechanism, because the portal code strikes me as
extremely crufty and unwieldy.  It may be that we can eventually remove
portals entirely, or perhaps reimplement them with this mechanism
underneath.

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