| Title | MPS AMC small nailed objects risk big retention by placement after big objects |
| Status | closed |
| Priority | optional |
| Assigned user | Richard Kistruck |
| Organization | Ravenbrook |
| Description | MPS AMC small nailed objects risk big retention by placement after big objects Related jobs: - job001809 "MPS ambiguous internal pointer makes AMC pool retain memory as pads" (fixes an earlier issue where a free-able pad was not being freed) RHSK 2009-04-03 After allocating a big object (which always starts its own segment) there may be some space left at the end of the MPS segment. If a small object is allocated there, and is then preserved by a legitimate, persistent, ambiguous reference, the MPS is unable to move it. The big object may either be preserved (by copy) or collected: either results in a large pad object. But the MPS cannot free the segment it is on, because of the small object. So that large useless pad persists, taking up memory and contributing nothing. |
| Analysis | RHSK 2008-05-01 BufferFill for very big object is rounded up, and this excess might then be used for a few small objects. This is bad strategy: if one of these small objects is nailed, it could retain the whole enormous segment. Very big objects should live on a segment by themselves. RHSK 2009-04-03 A solution would be to set apStruct.limit to just big enough for the requested big allocation. Any further allocation after that will trip, pad-out the rest, and BufferFill with a new segment. RHSK 2009-10-01 Fixed by treating any segment of at least AMCLargeSegPAGES as 'large', setting the allocation-point limit to precisely the mps_reserve() amount requested, and padding any remaining space in the last page of the segemnt -- this is LSP = Large Segment Padding, and the pad is an LSP pad. Gather statistics and emit diagnostics to show how well this policy is performing. |
| How found | inspection |
| Evidence | thinking about the code See also [pads on heap] < http://info.ravenbrook.com/mail/2008/04/28/18-39-11/0.txt>and the discussion that followed. |
| Observed in | 1.108.1 |
| Introduced in | 1.100.0 |
| Created by | Richard Kistruck |
| Created on | 2008-05-01 17:10:22 |
| Last modified by | Richard Kistruck |
| Last modified on | 2009-10-01 17:25:46 |
| History | 2008-05-01 RHSK Created. 2009-04-03 RHSK Propose solution. 2009-07-22 RHSK Link to job001809 2009-10-01 RHSK Write-up fix. |
| Change | Effect | Date | User | Description |
|---|---|---|---|---|
| 168704 | closed | 2009-09-18 16:15:50 | Richard Kistruck | MPS br/padding AMC LSP: rename PagesPerSegMediumLIMIT as AMCLargeSegPAGES, and make it a define in config.h. Corrections in des/poolamc. |
| 168688 | closed | 2009-09-17 16:59:39 | Richard Kistruck | MPS br/padding: poolamc.c tidy up: neater implementation of obj1pip (amcReclaimNailed) neater implementation of amcResetTraceIdStats -- no need for a function any more delete lots of obsolete temporary diagnostic (superseded by AMCTraceEnd_pageret) a few more avers (especially on buffer empty) also revert temporary diagnostic changes in arena.c, config.h, diag.c, global.c diag.c: fix diag-buffer at 100 screenfuls (200000 chars). zcoll.c: reinstate Make 50000 with occasional big objs. |
| 168661 | closed | 2009-09-14 17:12:43 | Richard Kistruck | MPS br/padding des/poolamc: complete documentaiton of AMC pads and retained pages. |
| 168478 | closed | 2009-08-20 17:44:16 | Richard Kistruck | MPS br/padding: new PoolTraceEndMethod: do end-of-trace work Tracer calls PoolTraceEnd() when the trace is TraceFINISHED. AbstractPoolClass uses PoolTrivTraceEnd -- a NOOP. [mpm.h, mpmst.h, mpmtypes.h, pool.c, poolabs.c] AMC overrides with AMCTraceEnd, to emit diagnostic on how well the trace went! [poolamc.c] |
| 168451 | closed | 2009-08-18 17:14:19 | Richard Kistruck | MPS br/padding zcoll.c: Change BigSmall() test to BigdropSmallambig() -- creates a big object and drops the ref to it, followed by a small object and keeps an ambig ref to it (to nail the AMCSeg it is in). Loops 100 times. This is for testing job001811 changes. |
| 167759 | open | 2009-04-08 16:04:09 | Richard Kistruck | MPS br/padding poolamc.c: change AMCBufferFill, so that for large requests (> 8 ArenaAligns) it gives only requested size; pad the rest. See job001811. The number 8 is provisional... |