diff --git a/lib/common/pool.c b/lib/common/pool.c
index 773488b07..281b3824a 100644
--- a/lib/common/pool.c
+++ b/lib/common/pool.c
@@ -10,9 +10,10 @@
 
 
 /* ======   Dependencies   ======= */
-#include <stddef.h>  /* size_t */
-#include "pool.h"
+#include <stddef.h>    /* size_t */
+#include "debug.h"     /* assert */
 #include "zstd_internal.h"  /* ZSTD_malloc, ZSTD_free */
+#include "pool.h"
 
 /* ======   Compiler specifics   ====== */
 #if defined(_MSC_VER)
@@ -33,8 +34,9 @@ typedef struct POOL_job_s {
 struct POOL_ctx_s {
     ZSTD_customMem customMem;
     /* Keep track of the threads */
-    ZSTD_pthread_t *threads;
-    size_t numThreads;
+    ZSTD_pthread_t* threads;
+    size_t threadCapacity;
+    size_t threadLimit;
 
     /* The queue is a circular buffer */
     POOL_job *queue;
@@ -58,10 +60,10 @@ struct POOL_ctx_s {
 };
 
 /* POOL_thread() :
-   Work thread for the thread pool.
-   Waits for jobs and executes them.
-   @returns : NULL on failure else non-null.
-*/
+ * Work thread for the thread pool.
+ * Waits for jobs and executes them.
+ * @returns : NULL on failure else non-null.
+ */
 static void* POOL_thread(void* opaque) {
     POOL_ctx* const ctx = (POOL_ctx*)opaque;
     if (!ctx) { return NULL; }
@@ -69,14 +71,17 @@ static void* POOL_thread(void* opaque) {
         /* Lock the mutex and wait for a non-empty queue or until shutdown */
         ZSTD_pthread_mutex_lock(&ctx->queueMutex);
 
-        while (ctx->queueEmpty && !ctx->shutdown) {
+        while ( ctx->queueEmpty
+            || (ctx->numThreadsBusy >= ctx->threadLimit) ) {
+            if (ctx->shutdown) {
+                /* even if !queueEmpty, (possible if numThreadsBusy >= threadLimit),
+                 * a few threads will be shutdown while !queueEmpty,
+                 * but enough threads will remain active to finish the queue */
+                ZSTD_pthread_mutex_unlock(&ctx->queueMutex);
+                return opaque;
+            }
             ZSTD_pthread_cond_wait(&ctx->queuePopCond, &ctx->queueMutex);
         }
-        /* empty => shutting down: so stop */
-        if (ctx->queueEmpty) {
-            ZSTD_pthread_mutex_unlock(&ctx->queueMutex);
-            return opaque;
-        }
         /* Pop a job off the queue */
         {   POOL_job const job = ctx->queue[ctx->queueHead];
             ctx->queueHead = (ctx->queueHead + 1) % ctx->queueSize;
@@ -89,30 +94,32 @@ static void* POOL_thread(void* opaque) {
             job.function(job.opaque);
 
             /* If the intended queue size was 0, signal after finishing job */
+            ZSTD_pthread_mutex_lock(&ctx->queueMutex);
+            ctx->numThreadsBusy--;
             if (ctx->queueSize == 1) {
-                ZSTD_pthread_mutex_lock(&ctx->queueMutex);
-                ctx->numThreadsBusy--;
-                ZSTD_pthread_mutex_unlock(&ctx->queueMutex);
                 ZSTD_pthread_cond_signal(&ctx->queuePushCond);
-        }   }
+            }
+            ZSTD_pthread_mutex_unlock(&ctx->queueMutex);
+        }
     }  /* for (;;) */
-    /* Unreachable */
+    assert(0);  /* Unreachable */
 }
 
 POOL_ctx* POOL_create(size_t numThreads, size_t queueSize) {
     return POOL_create_advanced(numThreads, queueSize, ZSTD_defaultCMem);
 }
 
-POOL_ctx* POOL_create_advanced(size_t numThreads, size_t queueSize, ZSTD_customMem customMem) {
+POOL_ctx* POOL_create_advanced(size_t numThreads, size_t queueSize,
+                               ZSTD_customMem customMem) {
     POOL_ctx* ctx;
-    /* Check the parameters */
+    /* Check parameters */
     if (!numThreads) { return NULL; }
     /* Allocate the context and zero initialize */
     ctx = (POOL_ctx*)ZSTD_calloc(sizeof(POOL_ctx), customMem);
     if (!ctx) { return NULL; }
     /* Initialize the job queue.
-     * It needs one extra space since one space is wasted to differentiate empty
-     * and full queues.
+     * It needs one extra space since one space is wasted to differentiate
+     * empty and full queues.
      */
     ctx->queueSize = queueSize + 1;
     ctx->queue = (POOL_job*)ZSTD_malloc(ctx->queueSize * sizeof(POOL_job), customMem);
@@ -126,7 +133,7 @@ POOL_ctx* POOL_create_advanced(size_t numThreads, size_t queueSize, ZSTD_customM
     ctx->shutdown = 0;
     /* Allocate space for the thread handles */
     ctx->threads = (ZSTD_pthread_t*)ZSTD_malloc(numThreads * sizeof(ZSTD_pthread_t), customMem);
-    ctx->numThreads = 0;
+    ctx->threadCapacity = 0;
     ctx->customMem = customMem;
     /* Check for errors */
     if (!ctx->threads || !ctx->queue) { POOL_free(ctx); return NULL; }
@@ -134,11 +141,12 @@ POOL_ctx* POOL_create_advanced(size_t numThreads, size_t queueSize, ZSTD_customM
     {   size_t i;
         for (i = 0; i < numThreads; ++i) {
             if (ZSTD_pthread_create(&ctx->threads[i], NULL, &POOL_thread, ctx)) {
-                ctx->numThreads = i;
+                ctx->threadCapacity = i;
                 POOL_free(ctx);
                 return NULL;
         }   }
-        ctx->numThreads = numThreads;
+        ctx->threadCapacity = numThreads;
+        ctx->threadLimit = numThreads;
     }
     return ctx;
 }
@@ -156,8 +164,8 @@ static void POOL_join(POOL_ctx* ctx) {
     ZSTD_pthread_cond_broadcast(&ctx->queuePopCond);
     /* Join all of the threads */
     {   size_t i;
-        for (i = 0; i < ctx->numThreads; ++i) {
-            ZSTD_pthread_join(ctx->threads[i], NULL);
+        for (i = 0; i < ctx->threadCapacity; ++i) {
+            ZSTD_pthread_join(ctx->threads[i], NULL);  /* note : could fail */
     }   }
 }
 
@@ -172,24 +180,68 @@ void POOL_free(POOL_ctx *ctx) {
     ZSTD_free(ctx, ctx->customMem);
 }
 
+
+
 size_t POOL_sizeof(POOL_ctx *ctx) {
     if (ctx==NULL) return 0;  /* supports sizeof NULL */
     return sizeof(*ctx)
         + ctx->queueSize * sizeof(POOL_job)
-        + ctx->numThreads * sizeof(ZSTD_pthread_t);
+        + ctx->threadCapacity * sizeof(ZSTD_pthread_t);
+}
+
+
+/* @return : 0 on success, 1 on error */
+static int POOL_resize_internal(POOL_ctx* ctx, size_t numThreads)
+{
+    if (numThreads <= ctx->threadCapacity) {
+        if (!numThreads) return 1;
+        ctx->threadLimit = numThreads;
+        return 0;
+    }
+    /* numThreads > threadCapacity */
+    {   ZSTD_pthread_t* const threadPool = (ZSTD_pthread_t*)ZSTD_malloc(numThreads * sizeof(ZSTD_pthread_t), ctx->customMem);
+        if (!threadPool) return 1;
+        /* replace existing thread pool */
+        memcpy(threadPool, ctx->threads, ctx->threadCapacity * sizeof(*threadPool));
+        ZSTD_free(ctx->threads, ctx->customMem);
+        ctx->threads = threadPool;
+        /* Initialize additional threads */
+        {   size_t threadId;
+            for (threadId = ctx->threadCapacity; threadId < numThreads; ++threadId) {
+                if (ZSTD_pthread_create(&threadPool[threadId], NULL, &POOL_thread, ctx)) {
+                    ctx->threadCapacity = threadId;
+                    return 1;
+            }   }
+    }   }
+    /* successfully expanded */
+    ctx->threadCapacity = numThreads;
+    ctx->threadLimit = numThreads;
+    return 0;
+}
+
+/* @return : 0 on success, 1 on error */
+int POOL_resize(POOL_ctx* ctx, size_t numThreads)
+{
+    int result;
+    if (ctx==NULL) return 1;
+    ZSTD_pthread_mutex_lock(&ctx->queueMutex);
+    result = POOL_resize_internal(ctx, numThreads);
+    ZSTD_pthread_cond_broadcast(&ctx->queuePopCond);
+    ZSTD_pthread_mutex_unlock(&ctx->queueMutex);
+    return result;
 }
 
 /**
  * Returns 1 if the queue is full and 0 otherwise.
  *
- * If the queueSize is 1 (the pool was created with an intended queueSize of 0),
- * then a queue is empty if there is a thread free and no job is waiting.
+ * When queueSize is 1 (pool was created with an intended queueSize of 0),
+ * then a queue is empty if there is a thread free _and_ no job is waiting.
  */
 static int isQueueFull(POOL_ctx const* ctx) {
     if (ctx->queueSize > 1) {
         return ctx->queueHead == ((ctx->queueTail + 1) % ctx->queueSize);
     } else {
-        return ctx->numThreadsBusy == ctx->numThreads ||
+        return (ctx->numThreadsBusy == ctx->threadLimit) ||
                !ctx->queueEmpty;
     }
 }
@@ -263,6 +315,11 @@ void POOL_free(POOL_ctx* ctx) {
     (void)ctx;
 }
 
+int POOL_resize(POOL_ctx* ctx, size_t numThreads) {
+    (void)ctx; (void)numThreads;
+    return 0;
+}
+
 void POOL_add(POOL_ctx* ctx, POOL_function function, void* opaque) {
     (void)ctx;
     function(opaque);
diff --git a/lib/common/pool.h b/lib/common/pool.h
index a57e9b4fa..458d37f13 100644
--- a/lib/common/pool.h
+++ b/lib/common/pool.h
@@ -30,40 +30,50 @@ typedef struct POOL_ctx_s POOL_ctx;
 */
 POOL_ctx* POOL_create(size_t numThreads, size_t queueSize);
 
-POOL_ctx* POOL_create_advanced(size_t numThreads, size_t queueSize, ZSTD_customMem customMem);
+POOL_ctx* POOL_create_advanced(size_t numThreads, size_t queueSize,
+                               ZSTD_customMem customMem);
 
 /*! POOL_free() :
-    Free a thread pool returned by POOL_create().
-*/
+ *  Free a thread pool returned by POOL_create().
+ */
 void POOL_free(POOL_ctx* ctx);
 
+/*! POOL_resize() :
+ *  Expands or shrinks pool's number of threads.
+ *  This is more efficient than releasing + creating a new context,
+ *  since it tries to preserve and re-use existing threads.
+ * `numThreads` must be at least 1.
+ * @return : 0 when resize was successful,
+ *           !0 (typically 1) if there is an error.
+ *    note : only numThreads can be resized, queueSize remains unchanged.
+ */
+int POOL_resize(POOL_ctx* ctx, size_t numThreads);
+
 /*! POOL_sizeof() :
-    return memory usage of pool returned by POOL_create().
-*/
+ * @return threadpool memory usage
+ *  note : compatible with NULL (returns 0 in this case)
+ */
 size_t POOL_sizeof(POOL_ctx* ctx);
 
 /*! POOL_function :
-    The function type that can be added to a thread pool.
-*/
+ *  The function type that can be added to a thread pool.
+ */
 typedef void (*POOL_function)(void*);
-/*! POOL_add_function :
-    The function type for a generic thread pool add function.
-*/
-typedef void (*POOL_add_function)(void*, POOL_function, void*);
 
 /*! POOL_add() :
-    Add the job `function(opaque)` to the thread pool. `ctx` must be valid.
-    Possibly blocks until there is room in the queue.
-    Note : The function may be executed asynchronously, so `opaque` must live until the function has been completed.
-*/
+ *  Add the job `function(opaque)` to the thread pool. `ctx` must be valid.
+ *  Possibly blocks until there is room in the queue.
+ *  Note : The function may be executed asynchronously,
+ *         therefore, `opaque` must live until function has been completed.
+ */
 void POOL_add(POOL_ctx* ctx, POOL_function function, void* opaque);
 
 
 /*! POOL_tryAdd() :
-    Add the job `function(opaque)` to the thread pool if a worker is available.
-    return immediately otherwise.
-   @return : 1 if successful, 0 if not.
-*/
+ *  Add the job `function(opaque)` to thread pool _if_ a worker is available.
+ *  Returns immediately even if not (does not block).
+ * @return : 1 if successful, 0 if not.
+ */
 int POOL_tryAdd(POOL_ctx* ctx, POOL_function function, void* opaque);
 
 
diff --git a/lib/compress/zstd_compress.c b/lib/compress/zstd_compress.c
index 87aa0eb03..0e05e488f 100644
--- a/lib/compress/zstd_compress.c
+++ b/lib/compress/zstd_compress.c
@@ -3647,13 +3647,9 @@ size_t ZSTD_compress_generic (ZSTD_CCtx* cctx,
         }
         if (params.nbWorkers > 0) {
             /* mt context creation */
-            if (cctx->mtctx == NULL || (params.nbWorkers != ZSTDMT_getNbWorkers(cctx->mtctx))) {
+            if (cctx->mtctx == NULL) {
                 DEBUGLOG(4, "ZSTD_compress_generic: creating new mtctx for nbWorkers=%u",
                             params.nbWorkers);
-                if (cctx->mtctx != NULL)
-                    DEBUGLOG(4, "ZSTD_compress_generic: previous nbWorkers was %u",
-                                ZSTDMT_getNbWorkers(cctx->mtctx));
-                ZSTDMT_freeCCtx(cctx->mtctx);
                 cctx->mtctx = ZSTDMT_createCCtx_advanced(params.nbWorkers, cctx->customMem);
                 if (cctx->mtctx == NULL) return ERROR(memory_allocation);
             }
diff --git a/lib/compress/zstdmt_compress.c b/lib/compress/zstdmt_compress.c
index cfb71de73..b180b1b7d 100644
--- a/lib/compress/zstdmt_compress.c
+++ b/lib/compress/zstdmt_compress.c
@@ -159,6 +159,25 @@ static void ZSTDMT_setBufferSize(ZSTDMT_bufferPool* const bufPool, size_t const
     ZSTD_pthread_mutex_unlock(&bufPool->poolMutex);
 }
 
+
+static ZSTDMT_bufferPool* ZSTDMT_expandBufferPool(ZSTDMT_bufferPool* srcBufPool, U32 nbWorkers)
+{
+    unsigned const maxNbBuffers = 2*nbWorkers + 3;
+    if (srcBufPool==NULL) return NULL;
+    if (srcBufPool->totalBuffers >= maxNbBuffers) /* good enough */
+        return srcBufPool;
+    /* need a larger buffer pool */
+    {   ZSTD_customMem const cMem = srcBufPool->cMem;
+        size_t const bSize = srcBufPool->bufferSize;   /* forward parameters */
+        ZSTDMT_bufferPool* newBufPool;
+        ZSTDMT_freeBufferPool(srcBufPool);
+        newBufPool = ZSTDMT_createBufferPool(nbWorkers, cMem);
+        if (newBufPool==NULL) return newBufPool;
+        ZSTDMT_setBufferSize(newBufPool, bSize);
+        return newBufPool;
+    }
+}
+
 /** ZSTDMT_getBuffer() :
  *  assumption : bufPool must be valid
  * @return : a buffer, with start pointer and size
@@ -309,6 +328,10 @@ static void ZSTDMT_freeSeqPool(ZSTDMT_seqPool* seqPool)
     ZSTDMT_freeBufferPool(seqPool);
 }
 
+static ZSTDMT_seqPool* ZSTDMT_expandSeqPool(ZSTDMT_seqPool* pool, U32 nbWorkers)
+{
+    return ZSTDMT_expandBufferPool(pool, nbWorkers);
+}
 
 
 /* =====   CCtx Pool   ===== */
@@ -354,6 +377,18 @@ static ZSTDMT_CCtxPool* ZSTDMT_createCCtxPool(unsigned nbWorkers,
     return cctxPool;
 }
 
+static ZSTDMT_CCtxPool* ZSTDMT_expandCCtxPool(ZSTDMT_CCtxPool* srcPool,
+                                              unsigned nbWorkers)
+{
+    if (srcPool==NULL) return NULL;
+    if (nbWorkers <= srcPool->totalCCtx) return srcPool;   /* good enough */
+    /* need a larger cctx pool */
+    {   ZSTD_customMem const cMem = srcPool->cMem;
+        ZSTDMT_freeCCtxPool(srcPool);
+        return ZSTDMT_createCCtxPool(nbWorkers, cMem);
+    }
+}
+
 /* only works during initialization phase, not during compression */
 static size_t ZSTDMT_sizeof_CCtxPool(ZSTDMT_CCtxPool* cctxPool)
 {
@@ -744,9 +779,9 @@ struct ZSTDMT_CCtx_s {
     ZSTD_CCtx_params params;
     size_t targetSectionSize;
     size_t targetPrefixSize;
-    roundBuff_t roundBuff;
+    int jobReady;        /* 1 => one job is already prepared, but pool has shortage of workers. Don't create a new job. */
     inBuff_t inBuff;
-    int jobReady;        /* 1 => one job is already prepared, but pool has shortage of workers. Don't create another one. */
+    roundBuff_t roundBuff;
     serialState_t serial;
     unsigned singleBlockingThread;
     unsigned jobIDMask;
@@ -797,6 +832,20 @@ static ZSTDMT_jobDescription* ZSTDMT_createJobsTable(U32* nbJobsPtr, ZSTD_custom
     return jobTable;
 }
 
+static size_t ZSTDMT_expandJobsTable (ZSTDMT_CCtx* mtctx, U32 nbWorkers) {
+    U32 nbJobs = nbWorkers + 2;
+    if (nbJobs > mtctx->jobIDMask+1) {  /* need more job capacity */
+        ZSTDMT_freeJobsTable(mtctx->jobs, mtctx->jobIDMask+1, mtctx->cMem);
+        mtctx->jobIDMask = 0;
+        mtctx->jobs = ZSTDMT_createJobsTable(&nbJobs, mtctx->cMem);
+        if (mtctx->jobs==NULL) return ERROR(memory_allocation);
+        assert((nbJobs != 0) && ((nbJobs & (nbJobs - 1)) == 0));  /* ensure nbJobs is a power of 2 */
+        mtctx->jobIDMask = nbJobs - 1;
+    }
+    return 0;
+}
+
+
 /* ZSTDMT_CCtxParam_setNbWorkers():
  * Internal use only */
 size_t ZSTDMT_CCtxParam_setNbWorkers(ZSTD_CCtx_params* params, unsigned nbWorkers)
@@ -963,6 +1012,24 @@ static ZSTD_CCtx_params ZSTDMT_initJobCCtxParams(ZSTD_CCtx_params const params)
     return jobParams;
 }
 
+
+/* ZSTDMT_resize() :
+ * @return : error code if fails, 0 on success */
+static size_t ZSTDMT_resize(ZSTDMT_CCtx* mtctx, unsigned nbWorkers)
+{
+    if (POOL_resize(mtctx->factory, nbWorkers)) return ERROR(memory_allocation);
+    CHECK_F( ZSTDMT_expandJobsTable(mtctx, nbWorkers) );
+    mtctx->bufPool = ZSTDMT_expandBufferPool(mtctx->bufPool, nbWorkers);
+    if (mtctx->bufPool == NULL) return ERROR(memory_allocation);
+    mtctx->cctxPool = ZSTDMT_expandCCtxPool(mtctx->cctxPool, nbWorkers);
+    if (mtctx->cctxPool == NULL) return ERROR(memory_allocation);
+    mtctx->seqPool = ZSTDMT_expandSeqPool(mtctx->seqPool, nbWorkers);
+    if (mtctx->seqPool == NULL) return ERROR(memory_allocation);
+    ZSTDMT_CCtxParam_setNbWorkers(&mtctx->params, nbWorkers);
+    return 0;
+}
+
+
 /*! ZSTDMT_updateCParams_whileCompressing() :
  *  Updates only a selected set of compression parameters, to remain compatible with current frame.
  *  New parameters will be applied to next compression job. */
@@ -979,15 +1046,6 @@ void ZSTDMT_updateCParams_whileCompressing(ZSTDMT_CCtx* mtctx, const ZSTD_CCtx_p
     }
 }
 
-/* ZSTDMT_getNbWorkers():
- * @return nb threads currently active in mtctx.
- * mtctx must be valid */
-unsigned ZSTDMT_getNbWorkers(const ZSTDMT_CCtx* mtctx)
-{
-    assert(mtctx != NULL);
-    return mtctx->params.nbWorkers;
-}
-
 /* ZSTDMT_getFrameProgression():
  * tells how much data has been consumed (input) and produced (output) for current frame.
  * able to count progression inside worker threads.
@@ -1088,15 +1146,7 @@ static size_t ZSTDMT_compress_advanced_internal(
     if (ZSTDMT_serialState_reset(&mtctx->serial, mtctx->seqPool, params))
         return ERROR(memory_allocation);
 
-    if (nbJobs > mtctx->jobIDMask+1) {  /* enlarge job table */
-        U32 jobsTableSize = nbJobs;
-        ZSTDMT_freeJobsTable(mtctx->jobs, mtctx->jobIDMask+1, mtctx->cMem);
-        mtctx->jobIDMask = 0;
-        mtctx->jobs = ZSTDMT_createJobsTable(&jobsTableSize, mtctx->cMem);
-        if (mtctx->jobs==NULL) return ERROR(memory_allocation);
-        assert((jobsTableSize != 0) && ((jobsTableSize & (jobsTableSize - 1)) == 0));  /* ensure jobsTableSize is a power of 2 */
-        mtctx->jobIDMask = jobsTableSize - 1;
-    }
+    CHECK_F( ZSTDMT_expandJobsTable(mtctx, nbJobs) );  /* only expands if necessary */
 
     {   unsigned u;
         for (u=0; u<nbJobs; u++) {
@@ -1221,12 +1271,15 @@ size_t ZSTDMT_initCStream_internal(
 {
     DEBUGLOG(4, "ZSTDMT_initCStream_internal (pledgedSrcSize=%u, nbWorkers=%u, cctxPool=%u)",
                 (U32)pledgedSrcSize, params.nbWorkers, mtctx->cctxPool->totalCCtx);
-    /* params are supposed to be fully validated at this point */
+
+    /* params supposed partially fully validated at this point */
     assert(!ZSTD_isError(ZSTD_checkCParams(params.cParams)));
     assert(!((dict) && (cdict)));  /* either dict or cdict, not both */
-    assert(mtctx->cctxPool->totalCCtx == params.nbWorkers);
 
     /* init */
+    if (params.nbWorkers != mtctx->params.nbWorkers)
+        CHECK_F( ZSTDMT_resize(mtctx, params.nbWorkers) );
+
     if (params.jobSize == 0) {
         params.jobSize = 1U << ZSTDMT_computeTargetJobLog(params);
     }
diff --git a/lib/compress/zstdmt_compress.h b/lib/compress/zstdmt_compress.h
index f79e3b441..4249a82de 100644
--- a/lib/compress/zstdmt_compress.h
+++ b/lib/compress/zstdmt_compress.h
@@ -126,11 +126,6 @@ size_t ZSTDMT_CCtxParam_setNbWorkers(ZSTD_CCtx_params* params, unsigned nbWorker
  *  New parameters will be applied to next compression job. */
 void ZSTDMT_updateCParams_whileCompressing(ZSTDMT_CCtx* mtctx, const ZSTD_CCtx_params* cctxParams);
 
-/* ZSTDMT_getNbWorkers():
- * @return nb threads currently active in mtctx.
- * mtctx must be valid */
-unsigned ZSTDMT_getNbWorkers(const ZSTDMT_CCtx* mtctx);
-
 /* ZSTDMT_getFrameProgression():
  * tells how much data has been consumed (input) and produced (output) for current frame.
  * able to count progression inside worker threads.
diff --git a/lib/decompress/zstd_decompress.c b/lib/decompress/zstd_decompress.c
index 148c87f9d..4de98a8e6 100644
--- a/lib/decompress/zstd_decompress.c
+++ b/lib/decompress/zstd_decompress.c
@@ -1108,15 +1108,11 @@ size_t ZSTD_execSequence(BYTE* op,
             return sequenceLength;
         }
         /* span extDict & currentPrefixSegment */
-        DEBUGLOG(2, "ZSTD_execSequence: found a 2-segments match")
         {   size_t const length1 = dictEnd - match;
-            DEBUGLOG(2, "first part (extDict) is %zu bytes long", length1);
             memmove(oLitEnd, match, length1);
             op = oLitEnd + length1;
             sequence.matchLength -= length1;
-            DEBUGLOG(2, "second part (prefix) is %zu bytes long", sequence.matchLength);
             match = prefixStart;
-            DEBUGLOG(2, "first byte of 2nd part : %02X", *prefixStart);
             if (op > oend_w || sequence.matchLength < MINMATCH) {
               U32 i;
               for (i = 0; i < sequence.matchLength; ++i) op[i] = match[i];
diff --git a/tests/poolTests.c b/tests/poolTests.c
index 00ee83015..6a058a5a3 100644
--- a/tests/poolTests.c
+++ b/tests/poolTests.c
@@ -15,11 +15,11 @@
 #include <stddef.h>
 #include <stdio.h>
 
-#define ASSERT_TRUE(p)                                                         \
-  do {                                                                         \
-    if (!(p)) {                                                                \
-      return 1;                                                                \
-    }                                                                          \
+#define ASSERT_TRUE(p)                                                       \
+  do {                                                                       \
+    if (!(p)) {                                                              \
+      return 1;                                                              \
+    }                                                                        \
   } while (0)
 #define ASSERT_FALSE(p) ASSERT_TRUE(!(p))
 #define ASSERT_EQ(lhs, rhs) ASSERT_TRUE((lhs) == (rhs))
@@ -32,10 +32,10 @@ struct data {
 
 void fn(void *opaque) {
   struct data *data = (struct data *)opaque;
-  pthread_mutex_lock(&data->mutex);
+  ZSTD_pthread_mutex_lock(&data->mutex);
   data->data[data->i] = data->i;
   ++data->i;
-  pthread_mutex_unlock(&data->mutex);
+  ZSTD_pthread_mutex_unlock(&data->mutex);
 }
 
 int testOrder(size_t numThreads, size_t queueSize) {
@@ -43,25 +43,26 @@ int testOrder(size_t numThreads, size_t queueSize) {
   POOL_ctx *ctx = POOL_create(numThreads, queueSize);
   ASSERT_TRUE(ctx);
   data.i = 0;
-  pthread_mutex_init(&data.mutex, NULL);
-  {
-    size_t i;
+  ZSTD_pthread_mutex_init(&data.mutex, NULL);
+  { size_t i;
     for (i = 0; i < 16; ++i) {
       POOL_add(ctx, &fn, &data);
     }
   }
   POOL_free(ctx);
   ASSERT_EQ(16, data.i);
-  {
-    size_t i;
+  { size_t i;
     for (i = 0; i < data.i; ++i) {
       ASSERT_EQ(i, data.data[i]);
     }
   }
-  pthread_mutex_destroy(&data.mutex);
+  ZSTD_pthread_mutex_destroy(&data.mutex);
   return 0;
 }
 
+
+/* --- test deadlocks --- */
+
 void waitFn(void *opaque) {
   (void)opaque;
   UTIL_sleepMilli(1);
@@ -72,8 +73,7 @@ int testWait(size_t numThreads, size_t queueSize) {
   struct data data;
   POOL_ctx *ctx = POOL_create(numThreads, queueSize);
   ASSERT_TRUE(ctx);
-  {
-    size_t i;
+  { size_t i;
     for (i = 0; i < 16; ++i) {
         POOL_add(ctx, &waitFn, &data);
     }
@@ -82,25 +82,177 @@ int testWait(size_t numThreads, size_t queueSize) {
   return 0;
 }
 
+
+/* --- test POOL_resize() --- */
+
+typedef struct {
+    ZSTD_pthread_mutex_t mut;
+    int val;
+    int max;
+    ZSTD_pthread_cond_t cond;
+} poolTest_t;
+
+void waitLongFn(void *opaque) {
+  poolTest_t* test = (poolTest_t*) opaque;
+  UTIL_sleepMilli(10);
+  ZSTD_pthread_mutex_lock(&test->mut);
+  test->val = test->val + 1;
+  if (test->val == test->max)
+    ZSTD_pthread_cond_signal(&test->cond);
+  ZSTD_pthread_mutex_unlock(&test->mut);
+}
+
+static int testThreadReduction_internal(POOL_ctx* ctx, poolTest_t test)
+{
+    int const nbWaits = 16;
+    UTIL_time_t startTime, time4threads, time2threads;
+
+    test.val = 0;
+    test.max = nbWaits;
+
+    startTime = UTIL_getTime();
+    {   int i;
+        for (i=0; i<nbWaits; i++)
+            POOL_add(ctx, &waitLongFn, &test);
+    }
+    ZSTD_pthread_mutex_lock(&test.mut);
+    ZSTD_pthread_cond_wait(&test.cond, &test.mut);
+    ASSERT_EQ(test.val, nbWaits);
+    ZSTD_pthread_mutex_unlock(&test.mut);
+    time4threads = UTIL_clockSpanNano(startTime);
+
+    ASSERT_EQ( POOL_resize(ctx, 2/*nbThreads*/) , 0 );
+    test.val = 0;
+    startTime = UTIL_getTime();
+    {   int i;
+        for (i=0; i<nbWaits; i++)
+            POOL_add(ctx, &waitLongFn, &test);
+    }
+    ZSTD_pthread_mutex_lock(&test.mut);
+    ZSTD_pthread_cond_wait(&test.cond, &test.mut);
+    ASSERT_EQ(test.val, nbWaits);
+    ZSTD_pthread_mutex_unlock(&test.mut);
+    time2threads = UTIL_clockSpanNano(startTime);
+
+    if (time4threads >= time2threads) return 1;   /* check 4 threads were effectively faster than 2 */
+    return 0;
+}
+
+static int testThreadReduction(void) {
+    int result;
+    poolTest_t test;
+    POOL_ctx* const ctx = POOL_create(4 /*nbThreads*/, 2 /*queueSize*/);
+
+    ASSERT_TRUE(ctx);
+
+    memset(&test, 0, sizeof(test));
+    ASSERT_FALSE( ZSTD_pthread_mutex_init(&test.mut, NULL) );
+    ASSERT_FALSE( ZSTD_pthread_cond_init(&test.cond, NULL) );
+
+    result = testThreadReduction_internal(ctx, test);
+
+    ZSTD_pthread_mutex_destroy(&test.mut);
+    ZSTD_pthread_cond_destroy(&test.cond);
+    POOL_free(ctx);
+
+    return result;
+}
+
+
+/* --- test abrupt ending --- */
+
+typedef struct {
+    ZSTD_pthread_mutex_t mut;
+    int val;
+} abruptEndCanary_t;
+
+void waitIncFn(void *opaque) {
+  abruptEndCanary_t* test = (abruptEndCanary_t*) opaque;
+  UTIL_sleepMilli(10);
+  ZSTD_pthread_mutex_lock(&test->mut);
+  test->val = test->val + 1;
+  ZSTD_pthread_mutex_unlock(&test->mut);
+}
+
+static int testAbruptEnding_internal(abruptEndCanary_t test)
+{
+    int const nbWaits = 16;
+
+    POOL_ctx* const ctx = POOL_create(3 /*numThreads*/, nbWaits /*queueSize*/);
+    ASSERT_TRUE(ctx);
+    test.val = 0;
+
+    {   int i;
+        for (i=0; i<nbWaits; i++)
+            POOL_add(ctx, &waitIncFn, &test);  /* all jobs pushed into queue */
+    }
+    ASSERT_EQ( POOL_resize(ctx, 1 /*numThreads*/) , 0 );   /* downsize numThreads, to try to break end condition */
+
+    POOL_free(ctx);  /* must finish all jobs in queue before giving back control */
+    ASSERT_EQ(test.val, nbWaits);
+    return 0;
+}
+
+static int testAbruptEnding(void) {
+    int result;
+    abruptEndCanary_t test;
+
+    memset(&test, 0, sizeof(test));
+    ASSERT_FALSE( ZSTD_pthread_mutex_init(&test.mut, NULL) );
+
+    result = testAbruptEnding_internal(test);
+
+    ZSTD_pthread_mutex_destroy(&test.mut);
+    return result;
+}
+
+
+
+/* --- test launcher --- */
+
 int main(int argc, const char **argv) {
   size_t numThreads;
+  (void)argc;
+  (void)argv;
+
+  if (POOL_create(0, 1)) {   /* should not be possible */
+    printf("FAIL: should not create POOL with 0 threads\n");
+    return 1;
+  }
+
   for (numThreads = 1; numThreads <= 4; ++numThreads) {
     size_t queueSize;
     for (queueSize = 0; queueSize <= 2; ++queueSize) {
+      printf("queueSize==%u, numThreads=%u \n",
+            (unsigned)queueSize, (unsigned)numThreads);
       if (testOrder(numThreads, queueSize)) {
         printf("FAIL: testOrder\n");
         return 1;
       }
+      printf("SUCCESS: testOrder\n");
       if (testWait(numThreads, queueSize)) {
         printf("FAIL: testWait\n");
         return 1;
       }
+      printf("SUCCESS: testWait\n");
     }
   }
-  printf("PASS: testOrder\n");
-  (void)argc;
-  (void)argv;
-  return (POOL_create(0, 1)) ? printf("FAIL: testInvalid\n"), 1
-                             : printf("PASS: testInvalid\n"), 0;
+
+  if (testThreadReduction()) {
+      printf("FAIL: thread reduction not effective \n");
+      return 1;
+  } else {
+      printf("SUCCESS: thread reduction effective (slower execution) \n");
+  }
+
+  if (testAbruptEnding()) {
+      printf("FAIL: jobs in queue not completed on early end \n");
+      return 1;
+  } else {
+      printf("SUCCESS: all jobs in queue completed on early end \n");
+  }
+
+  printf("PASS: all POOL tests\n");
+
   return 0;
 }
diff --git a/tests/zstreamtest.c b/tests/zstreamtest.c
index dc341d13f..c76622701 100644
--- a/tests/zstreamtest.c
+++ b/tests/zstreamtest.c
@@ -1647,13 +1647,13 @@ static int fuzzerTests_newAPI(U32 seed, U32 nbTests, unsigned startTest,
             DISPLAYLEVEL(5, "Creating new context \n");
             ZSTD_freeCCtx(zc);
             zc = ZSTD_createCCtx();
-            CHECK(zc==NULL, "ZSTD_createCCtx allocation error");
-            resetAllowed=0;
+            CHECK(zc == NULL, "ZSTD_createCCtx allocation error");
+            resetAllowed = 0;
         }
         if ((FUZ_rand(&lseed) & 0xFF) == 132) {
             ZSTD_freeDStream(zd);
             zd = ZSTD_createDStream();
-            CHECK(zd==NULL, "ZSTD_createDStream allocation error");
+            CHECK(zd == NULL, "ZSTD_createDStream allocation error");
             ZSTD_initDStream_usingDict(zd, NULL, 0);  /* ensure at least one init */
         }