DEBUGGING FEATURES FOR CLIENT OBJECTS
design.mps.object-debug
incomplete doc
pekka 1998-09-10
INTRODUCTION
.intro: This is the design for all the various debugging features that MPS
clients (and sometimes MPS developers) can use to discover what is happening to
their objects and the memory space.
.readership: MPS developers.
Document History
.hist.0: The first draft merely records all the various ideas about
fenceposting that came up in discussions in June, July and September 1998.
This includes the format wrapping idea from mail.ptw.1998-06-19.21-13(0).
pekka 1998-09-10
OVERVIEW
.over.fenceposts: In its current state, this document mostly talks about
fenceposts, straying a little into tagging where theses features have an effect
on each other. [There exist other documents that list other required features,
and propose interfaces and implementations. These will eventually be folded
into this one. pekka 1998-09-10]
REQUIREMENTS
.req.fencepost: Try to detect overwrites and underwrites of allocated blocks by
adding fenceposts (source req.product.??? VC++, req.epcore.fun.debug.support).
.req.fencepost.size: The fenceposts should be at least 4 bytes on either side
or 8 bytes if on one side only, with an adjustable content (although VC++ only
has 4 bytes with pattern 0xFDFDFDFD, having unwisely combined the
implementation with other debug features).
.req.fencepost.check: There should be a function to check all the fenceposts
(source req.epcore.fun.debug.support).
.req.free-block: Try to detect attempts to write and read free blocks.
.req.walk: There should be a way to map ("walk") a user function over all
allocated objects (except PS VM objects), possibly only in a separate debugging
variety/mode (source req.epcore.fun.debug.support).
.req.tag: There should be a way to store at least a word of user data (a "tag",
borrowing the SW term) with every object in debugging mode, to be used in
memory dumps (source req.product.??? VC++). .req.tag.walk: The walking
function (as required by .req.walk) should have access to this data (source
req.epcore.fun.debug.support).
.req.dump.aver: It must be possible to perform a memory dump after an AVER has
fired (naturally, if the information required for the dump has been corrupted,
it will fail, as softly as possible). (source @@@@)
[There are more, especially about memory dumps and allocation locations. pekka
1998-09-10]
SOLUTION IDEAS
.note.assumptions: I've tried not to assume anything about the coincidence of
manual/automatic, formatted/unformatted, and ap/mps_alloc. I think those
questions deserve to be decided on their own merits. instead of being
constrained by a debug feature.
.fence.content.repeat: The content of a fencepost could be specified as a
byte/word which used repeatedly to fill the fencepost.
.fence.content.template: The content could be given as a template which is of
the right size and is simply copied onto the fencepost.
.fence.walk: .req.fencepost.check requires the ability to find all the
allocated objects. In formatted pools, this is not a problem. In unformatted
pools, we could use the walker. It's a feasible strategy to bet that any pool
that might have to support fenceposting will also have a walking requirement.
.fence.tag: Fenceposting also needs to keep track which objects have
fenceposts. unless we manage to do them all. It would be easiest to put this
in the tags.
.fence.check.object: A function to check the fenceposts on a given object would
be nice.
.fence.ap: AP's could support fenceposting transparently by having a mode where
mps_reserve always goes out-of-line and fills in the fenceposts (the pool's
BufferFill method isn't involved). This would leave the MPS with more freedom
of implementation, especially when combined with some of the other ideas. We
think doing a function call for every allocation is not too bad for debugging.
.fence.outside-ap: We could also let the client insert their own fenceposts
outside the MPS allocation mechanism. Even if fenceposting were done like
this, we'd still want it to be an MPS feature, so we'd offer sample C macros
for adding the size of the fencepost and filling in the fencepost pattern.
Possibly something like this (while we could still store the parameters in the
pool or ap, there seems little point in doing so in this case, and having them
as explicit parameters to the macros allows the client to specify constants to
gain effiency):
#define mps_add_fencepost(size, fp_size)
#define mps_fill_fenceposts(obj, size, fp_size, fp_pattern)
The client would need to supply their own fencepost checking function,
obviously, but again we could offer one that matches the sample macros.
.fence.tail-only: In automatic pools, the presence of a fencepost at the head
of the allocated block results in the object reference being an internal
pointer. This means that the format or the pool would need to know about
fenceposting and convert between references and pointers. This would slow down
the critical path when fenceposting is used. This can be ameliorated by
putting a fencepost at the tail of
the block only: this obviates the internal pointer problem and could provide
almost the same degree of checking (provided the size was twice as large),
especially in copying pools, where there are normally no gaps between allocated
blocks. In addition to the inescapable effects on allocation and freeing
(including copying and reclaim thereunder), only scanning would have to know
about fenceposts. .fence.tail-only.under: Walking over all the objects in the
pool would be necessary to detect underwrites, as one couldn't be sure that
there is a fencepost before any given object (or where it's located exactly).
If the pool were doing the checking, it could be sure: it would know about
alignments and it could put fenceposts in padding objects (free blocks will
have them because they were once allocated) so there'd be one on either side of
any object (except at the head of a segment, which is not a major problem, and
could be fixed by adding a padding object at the beginning of every segment).
This requires some cleverness to avoid splinters smaller than the fencepost
size, but it can be done.
.fence.wrapper: On formatted pools, fenceposting could be implemented by
"wrapping" the client-supplied format at creation time. The wrapper can handle
the conversion from the fenceposted object and back. This will be invisible to
the client and gives the added benefit that the wrapper can validate fenceposts
on every format operation, should it desire. That is, the pool would see the
fenceposts as part of the client object, but the client would only see its
object; the format wrapper would translate between the two. Note that hiding
the fenceposts from scan methods, which are required to take a contiguous range
of objects, is a bit complicated. .fence.client-format: The MPS would supply
such a wrapper, but clients could also be allowed to write their own
fenceposted formats (provided they coordinate with allocation, see below).
This would make scanning fenceposted segments more efficient.
.fence.wrapper.variable: Furthermore, you could create different classes of
fencepost within a pool, because the fencepost itself could have a variable
format. For instance, you might choose to have the fencepost be minimal (1-2
words) for small objects, and more detailed/complex for large objects
(imagining that large objects are likely vector-ish and subject to overruns).
You could get really
fancy and have the fencepost class keyed to the object class (e.g., different
AP's create different classes of fenceposting).
.fence.wrapper.alloc: Even with a wrapped format, allocation and freeing would
still have know about the fenceposts. If allocation points are used, either
MPS-side (.fence.ap) or client-side (.fence.outside-ap) fenceposting could be
used, with the obvious modifications. .fence.wrapper.alloc.format: We could
add three format methods, to adjust the pointer and the size for alloc and
free, to put down the fenceposts during alloc, and to check them; to avoid
slowing down all allocation, this would require some MOPping to make the format
class affect the choice of the alloc and free methods (see
mail.pekka.1998-06-11.18-18). .fence.wrapper.alloc.size: We could just
communicate the size of the fenceposts between the format and the allocation
routines, but then you couldn't use variable fenceposts
(.fence.wrapper.variable). [All this applies to copying and reclaim
in a straight-forward manner, I think.]
.fence.pool.wrapper: Pools can be wrapped as well. This could be a natural way
to represent/implement the fenceposting changes to the Alloc and Free methods.
[@@@@alignment]
.fence.pool.new-class: We could simply offer a debugging version of each pool
class (e.g., mps_pool_class_mv_debug). As we have seen, debugging features
have synergies which make it advantageous to have a coordinated implementation,
so splitting them up would not just complicate the client interface, it would
also be an implementation problem; we can turn features on or off with pool
init parameters.
.fence.pool.abstract: We could simply use pool init parameters only to control
all debugging features (optargs would be useful here). While there migh be
subclasses and wrappers internally, the client would only see a single pool
class; in the internal view, this would be an abstract class, and the
parameters would determine which concrete class actually gets instantiated.
.tag.out-of-line: It would be nice if tags were stored out-of-line, so they can
be used to study allocation patterns and fragmentation behaviours. Such an
implementation of tagging could also easily be shared among several pools.
ARCHITECTURE
.pool: The implementation is at the pool level, because pools manage allocated
objects. A lot of the code will be generic, naturally, but the data structures
and the control interfaces attach to pools. In particular, clients will be
able to use tagging and fenceposting separately on each pool.
.fence.size: Having fenceposts of adjustable size and pattern is quite useful.
We feel that restricting the size to an integral multiple of the [pool or
format?] alignment is harmless and simplifies the implementation enormously.
.fence.template: We use templates (.fence.content.template) to fill in the
fenceposts, but we do not give any guarantees about the location of the
fenceposts, only that they're properly aligned. This leaves us the opportunity
to do tail-only fenceposting, if we choose.
.fence.slop: [see impl.c.dbgpool.FenceAlloc @@@@]
.fence.check.free: We check the fenceposts when freeing an object.
.unified-walk: Combine the walking and tagging requirements (.req.tag.walk and
@@@@) into a generic facility for walking and tagging objects with just one
interface and one name: tagging. Also combine the existing formatted object
walker into this metaphor, but allowing the format and tag parameters of the
step function be optional [this part has not been implemented yet pekka
1998-09-10].
.init: It simplifies the implementation of both tagging and fenceposting if
they are always on, so that we don't have to keep track of which objects have
been fenceposted and which have not, and don't have to have three kinds of
tags: for user data, for fenceposting, and for both. So we determine this at
pool init time (and let fenceposting turn on tagging, if necessary).
.pool-parameters: Fencepost templates and tag formats are passed in as pool
parameters.
.modularity: While a combined generic implementation of tags and fenceposts is
provided, it is structured so that each part of it could be implemented by a
pool-specific mechanism with a minimum of new protocol. [This will be
improved, when we figure out formatted pools -- they don't need tags for
fenceposting.]
.out-of-space: If there's no room for tags, we will not dip into the reservoir,
just fail to allocate the tag. If the alloc call had a reservoir permit, we
let it succeed even without a tag, and just make sure the free method will not
complain if it can't find a tag. If the call didn't have a reservoir permit,
we free the block allocated for the object and fail the allocation, so that the
client gets a chance
to do whatever low-memory actions they might want to do. [Should this depend
on whether there is anything in the reservoir?] This breaks the 1-to-1
relationship between tags and objects, so some checks cannot be made, but we do
count the "lost" tags.
[need to hash out how to do fenceposting in formatted pools]
Client interface
.interface.fenceposting:
.interface.fenceposting.check: A function to check all fenceposts in a pool
(AVERs if a problem is found)
void mps_pool_check_fenceposts(mps_pool_t pool);
[from here on, these are tentative and incomplete]
.interface.fenceposting.format: A function to wrap a format (class) to provide
fenceposting
mps_res_t mps_fmt_fencepost_wrap(mps_fmt_t *format_return,
mps_arena_t arena,
mps_fmt_t format,
[fp parameters])
.interface.fenceposting.add: A format method to adjust size of a block about to
be allocted to allow for fenceposts
void (*mps_fmt_adjust_fencepost_t)(size_t *size_io)
.interface.fenceposting.add: A format method to add a fencepost around a block
about to be allocated [the NULL method adds a tail fencepost]
void (*mps_fmt_put_fencepost_t)(mps_addr_t * addr_io, size_t size)
.interface.fenceposting.add: A format method to check the fenceposts around an
object [the NULL method checks tails]
mps_bool_t (*mps_fmt_check_fenceposts_t)(mps_addr_t)
.interface.fenceposting.pool: A function to wrap a pool class to provide
fenceposting (note absence of arena parameter)
mps_class_t mps_debug_class(mps_class_t class)
.interface.tags.alloc: Three functions to replace existing mps_alloc
(request.???.??? proposes to remove the varargs)
mps_res_t mps_alloc(mps_addr_t *, mps_pool_t, size_t);
mps_res_t mps_alloc_dbg(mps_addr_t *, mps_pool_t, size_t, ...);
mps_res_t mps_alloc_dbg_v(mps_addr_t *, mps_pool_t, size_t,
va_list);
.interface.tags.walker: Functions to walk all the allocated objects in a pool
or an arena (only client pools in this case), format and tag_data can be NULL
(tag_data really wants to be void *, not mps_addr_t, because it's stored
together with the internal tag data in an MPS internal pool)
void (*mps_objects_step_t)(mps_addr_t addr, size_t size, mps_fmt_t format,
mps_pool_t pool, void *tag_data, void *p)
void mps_pool_walk(mps_arena_t arena, mps_pool_t pool,
mps_objects_step_t step, void *p)
void mps_arena_walk(mps_arena_t arena, mps_objects_step_t step, void *p)
Examples
.example.debug-alloc:
#define MPS_ALLOC_DBG(res_io, addr_io, pool, size)
MPS_BEGIN
static mps_tag_A_s _ts = { __FILE__, __LINE__ };
*res_io = mps_alloc(addr_io, pool, size, _ts_)
MPS_END
IMPLEMENTATION
.new-pool: The client interface to control fenceposting consists of the new
classes mps_pool_class_mv_debug, mps_pool_class_epdl_debug, and
mps_pool_class_epdr_debug, and their new init parameter of type
mps_pool_debug_option_s. [This is a temporary solution, to get it out without
writing lots of new interface. pekka 1998-09-10]
.new-pool.impl: The debug pools are implemented using the "class wrapper"
EnsureDebugClass, which produces a subclass with modified init, finish, alloc,
and free methods. These methods are implemented in the generic debug class
code (impl.c.dbgpool), and are basically wrappers around the superclass methods
(invoked through the pool->class->super field). To find the data stored in the
class for the debugging features, they use the debugMixin method provided by
the subclass. So to make a debug subclass, three things should be provided: a
structure definition of the instance containing a PoolDebugMixinStruct, a pool
class function that uses
EnsureDebugClass, and a debugMixin method that locates the PoolDebugMixinStruct
within an instance.
.tags.splay: The tags are stored in a splay tree of tags allocated from a
subsidiary MFS pool. The client needs to specify the (maximum) size of the
client data in a tag, so that the pool can be created.
[Lots more should be said, eventually. pekka 1998-09-10]
| 2002-06-07 | RB | Converted from MMInfo database design document. |
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