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[PATCH 02/18] xfs: reduce the number of CIL lock round trips during comm

To: xfs@xxxxxxxxxxx
Subject: [PATCH 02/18] xfs: reduce the number of CIL lock round trips during commit
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 24 Sep 2010 22:31:00 +1000
In-reply-to: <1285331476-23015-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1285331476-23015-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

When commiting a transaction, we do a lock CIL state lock round trip
on every single log vector we insert into the CIL. This is resulting
in the lock being as hot as the inode and dcache locks on 8-way
create workloads. Rework the insertion loops to bring the number
of lock round trips to one per transaction for log vectors, and one
more do the busy extents.

Also change the allocation of the log vector buffer not to zero it
as we copy over the entire allocated buffer anyway.

This patch also includes a structural cleanup to the CIL item
insertion provided by Christoph Hellwig.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Alex Elder <aelder@xxxxxxx>
---
 fs/xfs/xfs_log_cil.c |  232 +++++++++++++++++++++++++++-----------------------
 1 files changed, 127 insertions(+), 105 deletions(-)

diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index 7e206fc..23d6ceb 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -146,102 +146,6 @@ xlog_cil_init_post_recovery(
 }
 
 /*
- * Insert the log item into the CIL and calculate the difference in space
- * consumed by the item. Add the space to the checkpoint ticket and calculate
- * if the change requires additional log metadata. If it does, take that space
- * as well. Remove the amount of space we addded to the checkpoint ticket from
- * the current transaction ticket so that the accounting works out correctly.
- *
- * If this is the first time the item is being placed into the CIL in this
- * context, pin it so it can't be written to disk until the CIL is flushed to
- * the iclog and the iclog written to disk.
- */
-static void
-xlog_cil_insert(
-       struct log              *log,
-       struct xlog_ticket      *ticket,
-       struct xfs_log_item     *item,
-       struct xfs_log_vec      *lv)
-{
-       struct xfs_cil          *cil = log->l_cilp;
-       struct xfs_log_vec      *old = lv->lv_item->li_lv;
-       struct xfs_cil_ctx      *ctx = cil->xc_ctx;
-       int                     len;
-       int                     diff_iovecs;
-       int                     iclog_space;
-
-       if (old) {
-               /* existing lv on log item, space used is a delta */
-               ASSERT(!list_empty(&item->li_cil));
-               ASSERT(old->lv_buf && old->lv_buf_len && old->lv_niovecs);
-
-               len = lv->lv_buf_len - old->lv_buf_len;
-               diff_iovecs = lv->lv_niovecs - old->lv_niovecs;
-               kmem_free(old->lv_buf);
-               kmem_free(old);
-       } else {
-               /* new lv, must pin the log item */
-               ASSERT(!lv->lv_item->li_lv);
-               ASSERT(list_empty(&item->li_cil));
-
-               len = lv->lv_buf_len;
-               diff_iovecs = lv->lv_niovecs;
-               IOP_PIN(lv->lv_item);
-
-       }
-       len += diff_iovecs * sizeof(xlog_op_header_t);
-
-       /* attach new log vector to log item */
-       lv->lv_item->li_lv = lv;
-
-       spin_lock(&cil->xc_cil_lock);
-       list_move_tail(&item->li_cil, &cil->xc_cil);
-       ctx->nvecs += diff_iovecs;
-
-       /*
-        * If this is the first time the item is being committed to the CIL,
-        * store the sequence number on the log item so we can tell
-        * in future commits whether this is the first checkpoint the item is
-        * being committed into.
-        */
-       if (!item->li_seq)
-               item->li_seq = ctx->sequence;
-
-       /*
-        * Now transfer enough transaction reservation to the context ticket
-        * for the checkpoint. The context ticket is special - the unit
-        * reservation has to grow as well as the current reservation as we
-        * steal from tickets so we can correctly determine the space used
-        * during the transaction commit.
-        */
-       if (ctx->ticket->t_curr_res == 0) {
-               /* first commit in checkpoint, steal the header reservation */
-               ASSERT(ticket->t_curr_res >= ctx->ticket->t_unit_res + len);
-               ctx->ticket->t_curr_res = ctx->ticket->t_unit_res;
-               ticket->t_curr_res -= ctx->ticket->t_unit_res;
-       }
-
-       /* do we need space for more log record headers? */
-       iclog_space = log->l_iclog_size - log->l_iclog_hsize;
-       if (len > 0 && (ctx->space_used / iclog_space !=
-                               (ctx->space_used + len) / iclog_space)) {
-               int hdrs;
-
-               hdrs = (len + iclog_space - 1) / iclog_space;
-               /* need to take into account split region headers, too */
-               hdrs *= log->l_iclog_hsize + sizeof(struct xlog_op_header);
-               ctx->ticket->t_unit_res += hdrs;
-               ctx->ticket->t_curr_res += hdrs;
-               ticket->t_curr_res -= hdrs;
-               ASSERT(ticket->t_curr_res >= len);
-       }
-       ticket->t_curr_res -= len;
-       ctx->space_used += len;
-
-       spin_unlock(&cil->xc_cil_lock);
-}
-
-/*
  * Format log item into a flat buffers
  *
  * For delayed logging, we need to hold a formatted buffer containing all the
@@ -286,7 +190,7 @@ xlog_cil_format_items(
                        len += lv->lv_iovecp[index].i_len;
 
                lv->lv_buf_len = len;
-               lv->lv_buf = kmem_zalloc(lv->lv_buf_len, KM_SLEEP|KM_NOFS);
+               lv->lv_buf = kmem_alloc(lv->lv_buf_len, KM_SLEEP|KM_NOFS);
                ptr = lv->lv_buf;
 
                for (index = 0; index < lv->lv_niovecs; index++) {
@@ -300,21 +204,136 @@ xlog_cil_format_items(
        }
 }
 
+/*
+ * Prepare the log item for insertion into the CIL. Calculate the difference in
+ * log space and vectors it will consume, and if it is a new item pin it as
+ * well.
+ */
+STATIC void
+xfs_cil_prepare_item(
+       struct log              *log,
+       struct xfs_log_vec      *lv,
+       int                     *len,
+       int                     *diff_iovecs)
+{
+       struct xfs_log_vec      *old = lv->lv_item->li_lv;
+
+       if (old) {
+               /* existing lv on log item, space used is a delta */
+               ASSERT(!list_empty(&lv->lv_item->li_cil));
+               ASSERT(old->lv_buf && old->lv_buf_len && old->lv_niovecs);
+
+               *len += lv->lv_buf_len - old->lv_buf_len;
+               *diff_iovecs += lv->lv_niovecs - old->lv_niovecs;
+               kmem_free(old->lv_buf);
+               kmem_free(old);
+       } else {
+               /* new lv, must pin the log item */
+               ASSERT(!lv->lv_item->li_lv);
+               ASSERT(list_empty(&lv->lv_item->li_cil));
+
+               *len += lv->lv_buf_len;
+               *diff_iovecs += lv->lv_niovecs;
+               IOP_PIN(lv->lv_item);
+
+       }
+
+       /* attach new log vector to log item */
+       lv->lv_item->li_lv = lv;
+
+       /*
+        * If this is the first time the item is being committed to the
+        * CIL, store the sequence number on the log item so we can
+        * tell in future commits whether this is the first checkpoint
+        * the item is being committed into.
+        */
+       if (!lv->lv_item->li_seq)
+               lv->lv_item->li_seq = log->l_cilp->xc_ctx->sequence;
+}
+
+/*
+ * Insert the log items into the CIL and calculate the difference in space
+ * consumed by the item. Add the space to the checkpoint ticket and calculate
+ * if the change requires additional log metadata. If it does, take that space
+ * as well. Remove the amount of space we addded to the checkpoint ticket from
+ * the current transaction ticket so that the accounting works out correctly.
+ */
 static void
 xlog_cil_insert_items(
        struct log              *log,
        struct xfs_log_vec      *log_vector,
-       struct xlog_ticket      *ticket,
-       xfs_lsn_t               *start_lsn)
+       struct xlog_ticket      *ticket)
 {
-       struct xfs_log_vec *lv;
-
-       if (start_lsn)
-               *start_lsn = log->l_cilp->xc_ctx->sequence;
+       struct xfs_cil          *cil = log->l_cilp;
+       struct xfs_cil_ctx      *ctx = cil->xc_ctx;
+       struct xfs_log_vec      *lv;
+       int                     len = 0;
+       int                     diff_iovecs = 0;
+       int                     iclog_space;
 
        ASSERT(log_vector);
+
+       /*
+        * Do all the accounting aggregation and switching of log vectors
+        * around in a separate loop to the insertion of items into the CIL.
+        * Then we can do a separate loop to update the CIL within a single
+        * lock/unlock pair. This reduces the number of round trips on the CIL
+        * lock from O(nr_logvectors) to O(1) and greatly reduces the overall
+        * hold time for the transaction commit.
+        *
+        * If this is the first time the item is being placed into the CIL in
+        * this context, pin it so it can't be written to disk until the CIL is
+        * flushed to the iclog and the iclog written to disk.
+        *
+        * We can do this safely because the context can't checkpoint until we
+        * are done so it doesn't matter exactly how we update the CIL.
+        */
+       for (lv = log_vector; lv; lv = lv->lv_next)
+               xfs_cil_prepare_item(log, lv, &len, &diff_iovecs);
+
+       /* account for space used by new iovec headers  */
+       len += diff_iovecs * sizeof(xlog_op_header_t);
+
+       spin_lock(&cil->xc_cil_lock);
+
+       /* move the items to the tail of the CIL */
        for (lv = log_vector; lv; lv = lv->lv_next)
-               xlog_cil_insert(log, ticket, lv->lv_item, lv);
+               list_move_tail(&lv->lv_item->li_cil, &cil->xc_cil);
+
+       ctx->nvecs += diff_iovecs;
+
+       /*
+        * Now transfer enough transaction reservation to the context ticket
+        * for the checkpoint. The context ticket is special - the unit
+        * reservation has to grow as well as the current reservation as we
+        * steal from tickets so we can correctly determine the space used
+        * during the transaction commit.
+        */
+       if (ctx->ticket->t_curr_res == 0) {
+               /* first commit in checkpoint, steal the header reservation */
+               ASSERT(ticket->t_curr_res >= ctx->ticket->t_unit_res + len);
+               ctx->ticket->t_curr_res = ctx->ticket->t_unit_res;
+               ticket->t_curr_res -= ctx->ticket->t_unit_res;
+       }
+
+       /* do we need space for more log record headers? */
+       iclog_space = log->l_iclog_size - log->l_iclog_hsize;
+       if (len > 0 && (ctx->space_used / iclog_space !=
+                               (ctx->space_used + len) / iclog_space)) {
+               int hdrs;
+
+               hdrs = (len + iclog_space - 1) / iclog_space;
+               /* need to take into account split region headers, too */
+               hdrs *= log->l_iclog_hsize + sizeof(struct xlog_op_header);
+               ctx->ticket->t_unit_res += hdrs;
+               ctx->ticket->t_curr_res += hdrs;
+               ticket->t_curr_res -= hdrs;
+               ASSERT(ticket->t_curr_res >= len);
+       }
+       ticket->t_curr_res -= len;
+       ctx->space_used += len;
+
+       spin_unlock(&cil->xc_cil_lock);
 }
 
 static void
@@ -638,7 +657,10 @@ xfs_log_commit_cil(
 
        /* lock out background commit */
        down_read(&log->l_cilp->xc_ctx_lock);
-       xlog_cil_insert_items(log, log_vector, tp->t_ticket, commit_lsn);
+       if (commit_lsn)
+               *commit_lsn = log->l_cilp->xc_ctx->sequence;
+
+       xlog_cil_insert_items(log, log_vector, tp->t_ticket);
 
        /* check we didn't blow the reservation */
        if (tp->t_ticket->t_curr_res < 0)
-- 
1.7.1

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