Server went down for a few hours. Why?

My server went down at lunchtime today and could only be restarted by a reboot. Looking at the logs, the following is repeated several times a second for a whle:

Nov 18 13:56:36 alpha kernel: Nov 18 13:56:36 alpha kernel: Mem-info: Nov 18 13:56:36 alpha kernel: DMA per-cpu: Nov 18 13:56:36 alpha kernel: cpu 0 hot: high 186, batch 31 used:169 Nov 18 13:56:36 alpha kernel: cpu 0 cold: high 62, batch 15 used:28 Nov 18 13:56:36 alpha kernel: cpu 1 hot: high 186, batch 31 used:33 Nov 18 13:56:36 alpha kernel: cpu 1 cold: high 62, batch 15 used:50 Nov 18 13:56:36 alpha kernel: DMA32 per-cpu: empty Nov 18 13:56:36 alpha kernel: Normal per-cpu: empty Nov 18 13:56:36 alpha kernel: HighMem per-cpu: empty Nov 18 13:56:36 alpha kernel: Free pages: 3820kB (0kB HighMem) Nov 18 13:56:36 alpha kernel: Active:117331 inactive:65552 dirty:0 writeback:0 unstable:0 free:955 slab:9267 mapped-file:1 mapped-anon:187110 pagetables:10769 Nov 18 13:56:36 alpha kernel: DMA free:3820kB min:3848kB low:4808kB high:5772kB active:469324kB inactive:262208kB present:925696kB pages_scanned:7572464 all_unreclaimable? yes Nov 18 13:56:36 alpha kernel: lowmem_reserve[]: 0 0 0 0 Nov 18 13:56:36 alpha kernel: DMA32 free:0kB min:0kB low:0kB high:0kB active:0kB inactive:0kB present:0kB pages_scanned:0 all_unreclaimable? no Nov 18 13:56:36 alpha kernel: lowmem_reserve[]: 0 0 0 0 Nov 18 13:56:36 alpha kernel: Normal free:0kB min:0kB low:0kB high:0kB active:0kB inactive:0kB present:0kB pages_scanned:0 all_unreclaimable? no Nov 18 13:56:36 alpha kernel: lowmem_reserve[]: 0 0 0 0 Nov 18 13:56:36 alpha kernel: HighMem free:0kB min:128kB low:128kB high:128kB active:0kB inactive:0kB present:0kB pages_scanned:0 all_unreclaimable? no Nov 18 13:56:36 alpha kernel: lowmem_reserve[]: 0 0 0 0 Nov 18 13:56:36 alpha kernel: DMA: 2834kB 1468kB 1316kB 132kB 064kB 0128kB 1256kB 0512kB 11024kB 02048kB 0*4096kB = 3820kB Nov 18 13:56:36 alpha kernel: DMA32: empty Nov 18 13:56:36 alpha kernel: Normal: empty Nov 18 13:56:36 alpha kernel: HighMem: empty Nov 18 13:56:36 alpha kernel: 36 pagecache pages Nov 18 13:56:36 alpha kernel: Swap cache: add 7004981, delete 7004974, find 18566623/19220363, race 5+564 Nov 18 13:56:36 alpha kernel: Free swap = 0kB Nov 18 13:56:36 alpha kernel: Total swap = 1835000kB Nov 18 13:56:36 alpha kernel: Free swap: 0kB Nov 18 13:56:36 alpha kernel: 231424 pages of RAM Nov 18 13:56:36 alpha kernel: 7321 reserved pages Nov 18 13:56:36 alpha kernel: 12885 pages shared Nov 18 13:56:36 alpha kernel: 7 pages swap cached Nov 18 13:56:36 alpha kernel: perl invoked oom-killer: gfp_mask=0x200d2, order=0, oomkilladj=0 Nov 18 13:56:36 alpha kernel: Nov 18 13:56:36 alpha kernel: Call Trace: Nov 18 13:56:36 alpha kernel: [] out_of_memory+0x8b/0x203 Nov 18 13:56:36 alpha kernel: [] __alloc_pages+0x245/0x2ce Nov 18 13:56:36 alpha kernel: [] __lock_page+0x5e/0x64 Nov 18 13:56:36 alpha kernel: [] read_swap_cache_async+0x42/0xd1 Nov 18 13:56:36 alpha kernel: [] swapin_readahead+0x4e/0x77 Nov 18 13:56:36 alpha kernel: [] __handle_mm_fault+0xcfc/0x11f6 Nov 18 13:56:36 alpha kernel: [] _spin_lock_irqsave+0x9/0x14 Nov 18 13:56:36 alpha kernel: [] do_page_fault+0xf7b/0x12e0 Nov 18 13:56:36 alpha kernel: [] hrtimer_cancel+0xc/0x16 Nov 18 13:56:36 alpha kernel: [] do_nanosleep+0x47/0x70 Nov 18 13:56:36 alpha kernel: [] hrtimer_nanosleep+0x58/0x118 Nov 18 13:56:36 alpha kernel: [] error_exit+0x0/0x6e Nov 18 13:56:36 alpha kernel: Nov 18 13:56:36 alpha kernel: Mem-info: Nov 18 13:56:36 alpha kernel: DMA per-cpu: Nov 18 13:56:36 alpha kernel: cpu 0 hot: high 186, batch 31 used:166 Nov 18 13:56:36 alpha kernel: cpu 0 cold: high 62, batch 15 used:28 Nov 18 13:56:36 alpha kernel: cpu 1 hot: high 186, batch 31 used:33 Nov 18 13:56:36 alpha kernel: cpu 1 cold: high 62, batch 15 used:50 Nov 18 13:56:36 alpha kernel: DMA32 per-cpu: empty

I have no idea what this means (but there are a lot of memory references), but assume the issue is visible to someone with more experience - can anybody help?