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mappings required by mdstart_swap(). On i386, if the ephemeral mapping is already in the sf_buf mapping cache, a swap-backed md performs similarly to a malloc-backed md. Even if the ephemeral mapping is not cached, this implementation is still faster. On 64-bit platforms, this change has the effect of using the direct virtual-to-physical mapping, avoiding ephemeral mapping overheads, such as TLB shootdowns on SMPs. On a 2.4GHz, 400MHz FSB P4 Xeon configured with 64K sf_bufs and "mdmfs -S -o async -s 128m md /mnt" before: dd if=/dev/md0 of=/dev/null bs=64k 134217728 bytes transferred in 0.430923 secs (311465697 bytes/sec) after with cold sf_buf cache: dd if=/dev/md0 of=/dev/null bs=64k 134217728 bytes transferred in 0.367948 secs (364773576 bytes/sec) after with warm sf_buf cache: dd if=/dev/md0 of=/dev/null bs=64k 134217728 bytes transferred in 0.252826 secs (530870010 bytes/sec) malloc-backed md: dd if=/dev/md0 of=/dev/null bs=64k 134217728 bytes transferred in 0.253126 secs (530240978 bytes/sec)
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