fix: cache ELF file descriptors per file, not per process#528
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fix: cache ELF file descriptors per file, not per process#528
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Benchmark results for collatzParameters
SummaryFound 0 performance improvements and 0 performance regressions! Performance is the same for 1 metrics, 0 unstable metrics. See unchanged results
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Benchmark results for BadBoggleSolver_runParameters
SummaryFound 0 performance improvements and 0 performance regressions! Performance is the same for 1 metrics, 0 unstable metrics. See unchanged results
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When profiling many forked processes sharing the same ELF files (e.g. libc), report_module() was opening a fresh fd for each process's Dwfl session. With N forks and M shared libraries, this resulted in N*M open file descriptors. Fix: add _fd_cache (unordered_map<FileInfoId_t, UniqueFd>) to DsoHdr, which is shared across all processes in UnwindState. The first time an ELF file is registered, it is opened and cached. Subsequent processes dup() the cached fd for dwfl ownership transfer. elfutils uses MAP_PRIVATE mmaps so the OS page cache already deduplicates physical pages; the dup() avoids redundant open() syscalls and fd exhaustion. Add STATS_ELF_FDS metric (elf.fds) to track the fd cache size each profiling cycle, observable in the notice-level stats output. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Mar 27, 2026
| // dup with O_CLOEXEC preserved: plain dup() drops close-on-exec, which | ||
| // would leak ELF fds into exec'd children. F_DUPFD_CLOEXEC atomically | ||
| // duplicates and sets FD_CLOEXEC, matching the original open(O_CLOEXEC). | ||
| UniqueFd fd_holder{::fcntl(cached_fd.get(), F_DUPFD_CLOEXEC, 0)}; |
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not very useful to add this (cloexec)
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What does this PR do?
When profiling many forked processes sharing the same ELF files (e.g. libc), report_module() was opening a fresh fd for each process's Dwfl session. With N forks and M shared libraries, this resulted in N*M open file descriptors.
Fix: add _fd_cache (unordered_map<FileInfoId_t, UniqueFd>) to DsoHdr, which is shared across all processes in UnwindState. The first time an ELF file is registered, it is opened and cached. Subsequent processes dup() the cached fd for dwfl ownership transfer. elfutils uses MAP_PRIVATE mmaps so the OS page cache already deduplicates physical pages; the dup() avoids redundant open() syscalls and fd exhaustion.
Add STATS_ELF_FDS metric (elf.fds) to track the fd cache size each profiling cycle, observable in the notice-level stats output.
Motivation
@bwoebi reported an issue with forking processes
Additional Notes
NA
How to test the change?
I straced a process that was forking before and after the fix: