"Zhijian Li (Fujitsu)" <[email protected]> writes: > 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.
@x-rdma-chunk-size is marked unstable, thus doesn't come with compatibility promises. Does unstable still make sense? > 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
