send_fileでダウンロードするとメモリ不足に？

ruby rails

小さなファイルならぜんぜん問題ないんですが、500MBくらいのファイルをsend_fileで送り出そうとすると、WEBrickプロセスが落ちます。

n 20 12:05:03 colinux kernel: oom-killer: gfp_mask=0x1d2
Jan 20 12:05:03 colinux kernel: DMA per-cpu: empty
Jan 20 12:05:04 colinux kernel: Normal per-cpu:
Jan 20 12:05:04 colinux kernel: cpu 0 hot: low 16, high 48, batch 8
Jan 20 12:05:04 colinux kernel: cpu 0 cold: low 0, high 16, batch 8
Jan 20 12:05:04 colinux kernel: HighMem per-cpu: empty
Jan 20 12:05:04 colinux kernel:
Jan 20 12:05:04 colinux kernel: Free pages:        1440kB (0kB HighMem)
Jan 20 12:05:04 colinux kernel: Active:28431 inactive:1417 dirty:0 writeback:0 unstable:0 free:360 slab:1030 mapped:28188 pagetables:143
Jan 20 12:05:04 colinux kernel: DMA free:0kB min:0kB low:0kB high:0kB active:0kB inactive:0kB present:0kB pages_scanned:0 all_unreclaimable? no
Jan 20 12:05:04 colinux kernel: protections[]: 0 0 0
Jan 20 12:05:04 colinux kernel: Normal free:1440kB min:1448kB low:1808kB high:2172kB active:113724kB inactive:5668kB present:131072kB pages_scanned:131976 all_unreclaimable? yes
Jan 20 12:05:04 colinux kernel: protections[]: 0 0 0
Jan 20 12:05:04 colinux kernel: HighMem free:0kB min:128kB low:160kB high:192kB active:0kB inactive:0kB present:0kB pages_scanned:0 all_unreclaimable? no
Jan 20 12:05:04 colinux kernel: protections[]: 0 0 0
Jan 20 12:05:04 colinux kernel: DMA: empty
Jan 20 12:05:04 colinux kernel: Normal: 0*4kB 0*8kB 0*16kB 1*32kB 0*64kB 1*128kB 1*256kB 0*512kB 1*1024kB 0*2048kB 0*4096kB = 1440kB
Jan 20 12:05:04 colinux kernel: HighMem: empty
Jan 20 12:05:04 colinux kernel: Swap cache: add 0, delete 0, find 0/0, race 0+0

:buffer_sizeとか:streamオプションをいじっても変わらりません。