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postcopy: Add doc about hugepages and postcopy 

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20170224182844.32452-16-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
This commit is contained in:
Dr. David Alan Gilbert 2017-02-24 18:28:43 +00:00
parent 7e8cafb713
commit 0c1f4036db
1 changed files with 13 additions and 0 deletions
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docs/migration.txt
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@ -540,3 +540,16 @@ request for a page that has already been sent is ignored. Duplicate requests
such as this can happen as a page is sent at about the same time the
destination accesses it.
=== Postcopy with hugepages ===
Postcopy now works with hugetlbfs backed memory:
a) The linux kernel on the destination must support userfault on hugepages.
b) The huge-page configuration on the source and destination VMs must be
identical; i.e. RAMBlocks on both sides must use the same page size.
c) Note that -mem-path /dev/hugepages will fall back to allocating normal
RAM if it doesn't have enough hugepages, triggering (b) to fail.
Using -mem-prealloc enforces the allocation using hugepages.
d) Care should be taken with the size of hugepage used; postcopy with 2MB
hugepages works well, however 1GB hugepages are likely to be problematic
since it takes ~1 second to transfer a 1GB hugepage across a 10Gbps link,
and until the full page is transferred the destination thread is blocked.