On Mon, Sep 07, 2026 at 03:51:12PM +0100, Daniel P. Berrangé wrote: > The migration code caches vmstate/ram writes into an iovec and > flushes this every 128kb.
I am just curious how did this 128K came from. Perhaps this? MAX_IOV_SIZE / 2 * 4K Where QEMU has: #define MAX_IOV_SIZE MIN_CONST(IOV_MAX, 64) And it needs to divides 2 because we always push save_page_header() first, which is 8B (in reality, maybe that'll also include some footers ahead from the last page..), then another 4K following it. Then in average when hitting 64 io vectors there're 32 pages, coming up to be that. I think that is right math for bulk ram phase, but maybe worth spelling out a bit.. because if above holds it's not very obvious.. > > The QIOChannelTLS receives the iovec, but size GNUTLS cannot > accept iovec data, it iterates calling send for each element. > > As a result of the migration data pattern, this results in > GNUTLS putting writes on the wire that alternate between about > 4k and 30 bytes. > > This is triggering the nagle algorithm on migration-test for > many of the TLS test cases, resulting in a "go slow" for I/O > that eventually hits the migration timeout configured by the > test. Worth spell out the qio_channel_set_delay() experiment? Frankly, even knowing qio_channel_set_delay(NO_DELAY) on all channels would fix it too, I don't think I fully get why the hang happened. Nagle, if my understanding is correct.. should be something trying to accumulate small writes only, it means write can be slightly delayed, but it didn't further explain why even if we push writting to it, it didn't flush properly. Say, I understand TLS is special now with its io_writev(), being qio_channel_tls_writev(), split the iov into multiple calls to qcrypto_tls_session_write(), which is likely why the problem existed, but I don't think I know why multiple qcrypto_tls_session_write() (and I believe ultimately, assuming small but continuous write()s to the socket fd) will cause a hang. Any clue? > > Not every contributor reports seeing the "go slow" but for > those who do see it, it hits >= 95% of the time for at least > one of the migration TLS test cases run by 'make check'. > > While we could disable the nagle algorithm (and multifd > channels already do this), that would just lead to lots of > small TCP packets hitting the wire which is not good for > throughput. The migration code flushes in batches of 128kb > because it wants large writes for high throughput. > > The only way to achieve this in the TLS code is to bounce > buffer the writes in order to flatten the iovec. We cannot > fully flatten, however, since GNUTLS puts a cap on the max > TLS record size it is willing to send, which is negotiated > with the server, typically 16 kb out of the box. > > This patch thus queries the max TLS record size and then > flattens the iovec into buffers of this size. If the > iovec only contains a single element, bounce buffering > is skipped to avoid the redundant copy. I saw there is also gnutls_record_cork() and the uncork(), which seems to resolve the same issue (I tried to look at gnutls git history but I didn't find any mention of why the API introduced.. though). Any thoughts on why not relying on that, say, would it work too if cork() at start of qio_channel_tls_writev(), loop, then uncork()? Thanks, > > Signed-off-by: Daniel P. Berrangé <[email protected]> > --- > crypto/tlssession.c | 10 ++++++++++ > include/crypto/tlssession.h | 2 ++ > include/io/channel-tls.h | 2 ++ > io/channel-tls.c | 35 ++++++++++++++++++++++++++++++----- > io/trace-events | 1 + > 5 files changed, 45 insertions(+), 5 deletions(-) > > diff --git a/crypto/tlssession.c b/crypto/tlssession.c > index 314e3e96ba..cf4daf8544 100644 > --- a/crypto/tlssession.c > +++ b/crypto/tlssession.c > @@ -653,6 +653,11 @@ qcrypto_tls_session_get_peer_name(QCryptoTLSSession > *session) > return NULL; > } > > +size_t qcrypto_tls_session_get_send_buffer(QCryptoTLSSession *session) > +{ > + return gnutls_record_get_max_size(session->handle); > +} > + > > #else /* ! CONFIG_GNUTLS */ > > @@ -757,4 +762,9 @@ qcrypto_tls_session_get_peer_name(QCryptoTLSSession *sess) > return NULL; > } > > +size_t qcrypto_tls_session_get_send_buffer(QCryptoTLSSession *sess) > +{ > + return 1; > +} > + > #endif > diff --git a/include/crypto/tlssession.h b/include/crypto/tlssession.h > index 28e419681e..b29648a0a9 100644 > --- a/include/crypto/tlssession.h > +++ b/include/crypto/tlssession.h > @@ -369,4 +369,6 @@ int qcrypto_tls_session_get_key_size(QCryptoTLSSession > *sess, > */ > char *qcrypto_tls_session_get_peer_name(QCryptoTLSSession *sess); > > +size_t qcrypto_tls_session_get_send_buffer(QCryptoTLSSession *sess); > + > #endif /* QCRYPTO_TLSSESSION_H */ > diff --git a/include/io/channel-tls.h b/include/io/channel-tls.h > index 7e9023570d..1c22a3fd07 100644 > --- a/include/io/channel-tls.h > +++ b/include/io/channel-tls.h > @@ -50,6 +50,8 @@ struct QIOChannelTLS { > QIOChannelShutdown shutdown; > guint hs_ioc_tag; > guint bye_ioc_tag; > + char *send_buffer; > + size_t send_buffer_len; > }; > > /** > diff --git a/io/channel-tls.c b/io/channel-tls.c > index 31ec4d236d..ba2699786e 100644 > --- a/io/channel-tls.c > +++ b/io/channel-tls.c > @@ -24,6 +24,7 @@ > #include "io/channel-tls.h" > #include "trace.h" > #include "qemu/atomic.h" > +#include "qemu/iov.h" > > > static ssize_t qio_channel_tls_write_handler(const void *buf, > @@ -201,6 +202,12 @@ static gboolean > qio_channel_tls_handshake_task(QIOChannelTLS *ioc, > } else { > trace_qio_channel_tls_credentials_allow(ioc); > } > + > + ioc->send_buffer_len = qcrypto_tls_session_get_send_buffer( > + ioc->session); > + ioc->send_buffer = g_new0(char, ioc->send_buffer_len); > + trace_qio_channel_tls_send_buffer_len(ioc, ioc->send_buffer_len); > + > qio_task_complete(task); > return TRUE; > } else { > @@ -376,6 +383,7 @@ static void qio_channel_tls_finalize(Object *obj) > g_clear_handle_id(&ioc->bye_ioc_tag, g_source_remove); > } > > + g_free(ioc->send_buffer); > object_unref(OBJECT(ioc->master)); > qcrypto_tls_session_free(ioc->session); > } > @@ -446,13 +454,30 @@ static ssize_t qio_channel_tls_writev(QIOChannel *ioc, > Error **errp) > { > QIOChannelTLS *tioc = QIO_CHANNEL_TLS(ioc); > - size_t i; > ssize_t done = 0; > + size_t tot = iov_size(iov, niov); > > - for (i = 0 ; i < niov ; i++) { > + /* Skip bounce buffer in simple case */ > + if (niov == 1) { > ssize_t ret = qcrypto_tls_session_write(tioc->session, > - iov[i].iov_base, > - iov[i].iov_len, > + iov[0].iov_base, > + iov[0].iov_len, > + errp); > + if (ret == QCRYPTO_TLS_SESSION_ERR_BLOCK) { > + return QIO_CHANNEL_ERR_BLOCK; > + } else if (ret < 0) { > + return -1; > + } > + return ret; > + } > + > + while (done < tot) { > + size_t got = iov_to_buf(iov, niov, done, > + tioc->send_buffer, > + tioc->send_buffer_len); > + ssize_t ret = qcrypto_tls_session_write(tioc->session, > + tioc->send_buffer, > + got, > errp); > if (ret == QCRYPTO_TLS_SESSION_ERR_BLOCK) { > if (done) { > @@ -464,7 +489,7 @@ static ssize_t qio_channel_tls_writev(QIOChannel *ioc, > return -1; > } > done += ret; > - if (ret < iov[i].iov_len) { > + if (ret < got) { > break; > } > } > diff --git a/io/trace-events b/io/trace-events > index ec91453335..88ebd5b478 100644 > --- a/io/trace-events > +++ b/io/trace-events > @@ -44,6 +44,7 @@ qio_channel_tls_handshake_pending(void *ioc, int status) > "TLS handshake pending > qio_channel_tls_handshake_fail(void *ioc) "TLS handshake fail ioc=%p" > qio_channel_tls_handshake_complete(void *ioc) "TLS handshake complete ioc=%p" > qio_channel_tls_handshake_cancel(void *ioc) "TLS handshake cancel ioc=%p" > +qio_channel_tls_send_buffer_len(void *ioc, int len) "TLS send buffer len > ioc=%p len=%d" > qio_channel_tls_bye_start(void *ioc) "TLS termination start ioc=%p" > qio_channel_tls_bye_pending(void *ioc, int status) "TLS termination pending > ioc=%p status=%d" > qio_channel_tls_bye_fail(void *ioc) "TLS termination fail ioc=%p" > -- > 2.55.0 > -- Peter Xu
