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Type:
Improvement
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Resolution: Won't Fix
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Priority:
Major - P3
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None
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Affects Version/s: None
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Component/s: None
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Labels:None
WT-4571 added 2 read barriers in transaction getting its snapshot (__wt_txn_get_snapshot) for safety reason. This ticket is to investigate the possibility of eliminating the read barriers.
150 void
151 __wt_txn_get_snapshot(WT_SESSION_IMPL *session)
152 {
...
217 while (s != txn_state &&
218 (id = s->id) != WT_TXN_NONE &&
219 WT_TXNID_LE(prev_oldest_id, id) &&
220 WT_TXNID_LT(id, current_id)) {
221 /*
222 * If the transaction is still allocating its ID, then
223 * we spin here until it gets its valid ID.
224 */
225 WT_READ_BARRIER();
226 if (!s->is_allocating) {
227 /*
228 * There is still a chance that fetched ID is
229 * not valid after ID allocation, so we check
230 * again here. The read of transaction ID
231 * should be carefully ordered: we want to
232 * re-read ID from transaction state after this
233 * transaction completes ID allocation.
234 */
235 WT_READ_BARRIER();
236 if (id == s->id) {
237 txn->snapshot[n++] = id;
238 if (WT_TXNID_LT(id, pinned_id))
239 pinned_id = id;
240 break;
241 }
242 }
243 WT_PAUSE();
244 }
The related discussion can be found on this github conversation: https://github.com/wiredtiger/wiredtiger/pull/4485#discussion_r261035635
Note that this change can also be applied to __txn_oldest_scan which has also 2 read barriers.