Samuel,
Thanks for the patch. On 14/05/2026 11:18, Samuel Zhang wrote: > The 1MiB default dates back to the original RDMA implementation in > 2013 (commit 2da776db48), and is too conservative for modern hardware. > > 64MiB captures most of the throughput gain (~10x over 1MiB) while > keeping transferred data low. Larger chunks cause more data to be > retransferred per dirty page, so the largest chunk size is not > necessarily optimal (see 1024MiB row). The x-rdma-chunk-size > parameter remains available for user tuning. > > Test config: BlueField-3 ConnectX-7, 8GB VM RAM, pin-all off, > `stress-ng --vm 4 --vm-bytes 1G --vm-method rand-set` > > chunk_size total(ms) down(ms) Throughput(Mbps) transferred > 1m 45,156 1,166 1,252.50 6.46 GiB > 32m 15,034 1,864 3,401.26 5.57 GiB > 64m 4,492 1,554 13,637.46 5.75 GiB > 128m 3,940 1,662 16,860.59 6.06 GiB > 1024m 3,665 2,238 24,676.59 8.04 GiB > > Signed-off-by: Samuel Zhang <[email protected]> > --- > migration/options.c | 2 +- > qapi/migration.json | 2 +- > 2 files changed, 2 insertions(+), 2 deletions(-) > > diff --git a/migration/options.c b/migration/options.c > index 5cbfd29099..ea2137372c 100644 > --- a/migration/options.c > +++ b/migration/options.c > @@ -91,7 +91,7 @@ const PropertyInfo qdev_prop_StrOrNull; > > #define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT_PERIOD 1000 /* milliseconds > */ > #define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT 1 /* MB/s */ > -#define DEFAULT_MIGRATE_X_RDMA_CHUNK_SIZE MiB > +#define DEFAULT_MIGRATE_X_RDMA_CHUNK_SIZE (64 * MiB) I have a concern about backward compatibility. AFAIK, changing the default chunk size could break RDMA migration between hosts running different QEMU versions. If this happens, the error message is not clear enough for a user to understand that the failure is due to a mismatch in 'x-rdma-chunk-size'? [1] https://lore.kernel.org/qemu-devel/[email protected]/ Thanks Zhijian > > const Property migration_properties[] = { > DEFINE_PROP_BOOL("store-global-state", MigrationState, > diff --git a/qapi/migration.json b/qapi/migration.json > index 0db115ec5e..b3815f0594 100644 > --- a/qapi/migration.json > +++ b/qapi/migration.json > @@ -1009,7 +1009,7 @@ > # the remainder its arguments. (Since 10.2) > # > # @x-rdma-chunk-size: RDMA memory registration chunk size in bytes. > -# Default is 1MiB. Must be a power of 2 in the range > +# Default is 64MiB. Must be a power of 2 in the range > # [1MiB, 1024MiB]. Only applies when migrating via RDMA. > # Must be set to the same value on both source and destination > # before migration starts. (Since 11.1)
