Bug 253877 - [libgeom] O(n^2) behavior in geom_stats_resync
Summary: [libgeom] O(n^2) behavior in geom_stats_resync
Status: In Progress
Alias: None
Product: Base System
Classification: Unclassified
Component: bin (show other bugs)
Version: CURRENT
Hardware: Any Any
: --- Affects Many People
Assignee: Alan Somers
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2021-02-26 17:33 UTC by Alan Somers
Modified: 2021-03-04 00:36 UTC (History)
1 user (show)

See Also:
asomers: mfc-stable13?
asomers: mfc-stable12?


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Description Alan Somers freebsd_committer 2021-02-26 17:33:31 UTC
geom_stats_resync tries to mmap all of /dev/devstat.  But without knowing that device's size, it simply tries to mmap successively larger amounts of memory until it fails.  That means that the entire operation has O(n^2) syscalls, where n is related to the number of geom providers.  On a system with a few thousand providers, running "mdconfig -l" can take several minutes.

void
geom_stats_resync(void)
{
        void *p;

        if (statsfd == -1)
                return;
        for (;;) {
                p = mmap(statp, (npages + 1) * pagesize,
                    PROT_READ, MAP_SHARED, statsfd, 0);
                if (p == MAP_FAILED)
                        break;
                else
                        statp = p;
                npages++;
        }
}
Comment 1 Alan Somers freebsd_committer 2021-03-04 00:36:37 UTC
Fixed by this commit, but I forgot to include the PR tag.
https://cgit.freebsd.org/src/commit/?id=ab63da3564e8ab0907f9d8eb565774848ffdadeb