40. Tracer

40.1. Introduction

Warning

This document is currently a mixture of very old design notes (the preformatted section immediately following) and some newer stuff. It doesn’t yet form anything like a complete picture.

40.2. Architecture

.instance.limit: There is a limit on the number of traces that can be created at any one time. This limits the number of concurrent traces. This limitation is expressed in the symbol TraceLIMIT.

Note

TraceLIMIT is currently set to 1 as the MPS assumes in various places that only a single trace is active at a time. See request.mps.160020 “Multiple traces would not work”. David Jones, 1998-06-15.

.rate: See mail.nickb.1997-07-31.14-37.

Note

Now revised? See request.epcore.160062 and change.epcore.minnow.160062. David Jones, 1998-06-15.

.exact.legal: Exact references must either point outside the arena (to non-managed address space) or to a tract allocated to a pool. Exact references that are to addresses which the arena has reserved but hasn’t allocated memory to are illegal (such a reference cannot possibly refer to a real object, and so cannot be exact). We check that this is the case in TraceFix().

Note

Depending on the future semantics of PoolDestroy() we might need to adjust our strategy here. See mail.dsm.1996-02-14.18-18 for a strategy of coping gracefully with PoolDestroy().

.fix.fixed.all: ss->fixedSummary is accumulated (in TraceFix()) for all pointers, whether or not they are genuine references. We could accumulate fewer pointers here; if a pointer fails the TractOfAddr() test then we know it isn’t a reference, so we needn’t accumulate it into the fixed summary. The design allows this, but it breaks a useful post-condition on scanning (if the accumulation of ss->fixedSummary was moved the accuracy of ss->fixedSummary would vary according to the “width” of the white summary). See mail.pekka.1998-02-04.16-48 for improvement suggestions.

40.3. Analysis

.fix.copy-fail: Fixing can always succeed, even if copying the referenced object has failed (due to lack of memory, for example), by backing off to treating a reference as ambiguous. Assuming that fixing an ambiguous reference doesn’t allocate memory (which is no longer true for AMC for example). See request.dylan.170560 for a slightly more sophisticated way to proceed when you can no longer allocate memory for copying.

40.4. Ideas

.flip.after: To avoid excessive barrier impact on the mutator immediately after flip, we could scan during flip other objects which are “near” the roots, or otherwise known to be likely to be accessed in the near future.

40.5. Implementation

40.5.1. Speed

.fix: The function implementing the fix operation should be called TraceFix() and this name is pervasive in the MPS and its documents to describe this function. Nonethless, optimisation and strict aliasing rules have meant that we need to use the external name for it, _mps_fix2().

.fix.speed: The fix path is critical to garbage collection speed. Abstractly, the fix operation is applied to all references in the non-white heap and all references in the copied heap. Remembered sets cut down the number of segments we have to scan. The zone test cuts down the number of references we call fix on. The speed of the remainder of the fix path is still critical to system performance. Various modifications to and aspects of the system are concerned with maintaining the speed along this path. See design.mps.critical_path.

.fix.tractofaddr: A reference that passes the zone test is then looked up to find the tract it points to, an operation equivalent to calling TractOfAddr().

.fix.tractofaddr.inline: TraceFix() doesn’t actually call TractOfAddr(). Instead, it expands this operation inline (calling ChunkOfAddr(), then INDEX_OF_ADDR(), checking the appropriate bit in the chunk’s allocTable, and finally looking up the tract in the chunk’s page table). The reason for inlining this code is that we need to know whether the reference points to a chunk (and not just whether it points to a tract) in order to check the .exact.legal condition.

.fix.whiteseg: The reason for looking up the tract is to determine whether the segment is white. There is no need to examine the segment to perform this test, since whiteness information is duplicated in tracts, specifically to optimize this test.

Note

Nonetheless, it is likely to be more efficient to maintain a separate lookup table from address to white segment, rather than indirecting through the chunk and the tract. See job003796.

.fix.noaver: AVER() statements in the code add bulk to the code (reducing I-cache efficacy) and add branches to the path (polluting the branch pedictors) resulting in a slow down. Replacing the AVER() statements with AVER_CRITICAL() on the critical path improves the overall speed of the Dylan compiler by as much as 9%. See design.mps.critical_path.

.fix.nocopy: AMCFix() used to copy objects by using the format’s copy method. This involved a function call (through an indirection) and in dylan_copy a call to dylan_skip (to recompute the length) and call to memcpy with general parameters. Replacing this with a direct call to memcpy removes these overheads and the call to memcpy now has aligned parameters. The call to memcpy is inlined by the C compiler. This change results in a 4–5% speed-up in the Dylan compiler.

.reclaim: Because the reclaim phase of the trace (implemented by TraceReclaim()) examines every segment it is fairly time intensive. Richard Tucker’s profiles presented in request.dylan.170551 show a gap between the two varieties variety.hi and variety.wi.

.reclaim.noaver: Accordingly, reclaim methods use AVER_CRITICAL() instead of AVER().

40.6. Life cycle of a trace object

TraceCreate() creates a trace in state TraceINIT

Some segments get condemned (made white).

TraceStart() gets called which:

  • Derives an initial reference partition based on the existing white set. The white zone set and the segments’ summaries are used to create an initial grey set.

  • Emits a GCStart() message.

  • Initialises trace->rate by estimating the required scanning rate.

  • Moves the trace into the state TraceUNFLIPPED.

  • Immediately calls traceFlip which flips the trace and moves it into state TraceFLIPPED.

Whilst a trace is alive every so often its TraceAdvance() method gets invoked (via TracePoll()) in order to do a step of tracing work. TraceAdvance() is responsible for ticking through the trace’s top-level state machine. Most of the interesting work, the tracing, happens in the TraceFLIPPED state.

The trace transitions through its states in the following sequence: TraceINIT → (TraceUNFLIPPED) → TraceFLIPPED → TraceRECLAIM → TraceFINISHED.

Whilst TraceUNFLIPPED appears in the code, no trace does any work in this state; all traces are immediately flipped to be in the TraceFLIPPED state (see above).

Once the trace is in the TraceFINISHED state it performs no more work and it can be safely destroyed. Generally the callers of TraceAdvance() will destroy the trace.

40.6.1. Making progress: scanning grey segments

Most of the interesting work of a trace, the actual tracing, happens in the TraceFLIPPED state (work would happen in the TraceUNFLIPPED state, but that is not implemented).

The tracer makes progress by choosing a grey segment to scan, and scanning it. The actual scanning is performed by pools.

Note that at all times a reference partition is maintained.

The order in which the trace scans things determines the semantics of certain types of references (in particular, weak and final references). Or, to put it another way the desired semantics of weak and final references impose certain restrictions on the order in which the trace can scan things.

The tracer uses a system of reference ranks (or just ranks) so that it can impose an order on its scanning work. The ranks are ordered.

The tracer proceeds band by band. The first band is all objects it can reach by following references of the first rank. The second band is all subsequent objects it can reach by following references of the second and first ranks. The third band is all subsequent objects it can reach by following references of the third, second, and first ranks. And so on. The description of the tracer working like this originated in [RHSK_2007-06-25].

A trace keeps track of which band it is tracing. This is returned by the TraceBand() method. Keeping this band information helps it implement the semantics of finalization and weakness. The band used to not be explicitly stored, but this hindered the implementation of good finalization semantics (in some circumstances finalization messages were delayed by at least one collection cycle: see job001658).

The band is used when selecting a grey segment to scan (the selection occurs in traceFindGrey()). The tracer attempts to first find segments whose rank is the current band, then segments whose rank is previous to the current band, and so on. If there are no segments found then the current band is exhausted and the current band is incremented to the next rank. When the current band is moved through all the ranks in this fashion there is no more tracing to be done.

40.7. References

RHSK_2007-06-25

Richard Kistruck. Ravenbrook Limited. 2007-06-25. “The semantics of rank-based tracing”.